How can I improve my middlegame in chess?

I’ve been trying to improve my middlegame in chess, but I feel like I’m stuck.

Do you have any tips, resources, or exercises you could recommend? I’d really appreciate any advice you have!

Thanks a bunch!

Thank you, so first I will develop my pieces and then castle

Learn and apply the most important principles of chess. - (core of my teaching)
Always blunder-check your moves.
Solve tactics in the right way.
Analyze your games.
Study games of strong players.
Learn how to be more psychologically resilient.
Work on your time management skills.
Get a coach if you can.

Depends on where you think your weak points are? Are you overlooking pieces under attack? Are you getting hit with pins and forks from your opponent that you didn't see? Are you getting to a decent position out of the opening and then having no idea what to do next?

If the first, then take more time going over the board, looking for your undefended pieces, and looking at what pieces your opponent is attacking now. The second is similar, but you look to see what your opponent could do after a move, so not what the board is right now but what the board would be if your opponent gets to make a move instead of you. The third is trickier. If I find myself in a position where things look good, but there isn't any obvious tactics available, then I sometimes find my thinking just collapses and I have no idea what to do next. I can find myself desperately looking for an attacking move to try and force something to happen, thinking that I'm just missing the winning blow. That is almost never the case though.

What I'm working on in those situations is to take a moment, and start looking for less immediate things but rather ideas that might require a bit of time to achieve, such as are there any outpost squares for me? If so, how can I get a Knight there? Or, I have a light square Bishop and they have a dark square Bishop, so I should start trying to put pawns on dark squares. Or, their Knight/Bishop/Rook, etc is a bit stuck and not doing a great deal, what squares does it need to get to in order to get back into the game? Can I control those squares?
In other words, in the middle game, after a successful opening, it may be that the next phase is more about implementing some ideas about how to make your position better while making your opponent's position a bit harder. Eventually, those ideas will lead to tactical opportunities.

Anyway, it also may just be that you've reached a temporary plateau, and as you fine tune the information and knowledge you've learned, it will eventually calibrate itself properly and you'll start your next climb.

What do you mean when you say "learn to be more psychologically resilient"?

And where can I get a trainer?

Ohhhh I understand, thank you very much, but how can I better understand the position and look for some ideas like the ones you mention, without making mistakes in the attempt? some tips?

Learn strategies based on PAWN STRUCTURES & learn basic endings thoroughly.

Thank you for the valuable advice! 
I truly appreciate your guidance on this, as understanding these elements can dramatically improve one’s strategic depth and overall game. Your insights have given me a clear direction for what to prioritize in my studies, and I am grateful for that.

Puzzles.

Thank you bro

Like most things conceptual, the more you work on the concept the more clear it eventually becomes! I know, not very helpful.
So outpost squares are squares in front of your opponent's pawn but that your opponent can no longer attack with a pawn (so, put a white pawn on e4 and d3, and remove White's c-pawn, or put it on c4. That makes d4 a potential outpost for Black. If you can get a Knight there, that tends to really improve the value of that Knight, particularly if you are able to defend your Knight with a pawn (i.e. if you have a pawn on c5). That Knight might not immediately be attacking anything, there might not be a tactic just yet, but having that Knight will be a thorn in your opponent's side that will hinder their piece movement and blocks their pawns from advancing. And it isn't easily attacked, so it is hard for them to dislodge.
Things like getting your pawns to the opposite colour of your one remaining Bishop (particularly if your opponent only has the other one, so opposite coloured Bishops situations), does two things. One, your Bishop can already cover things on its coloured squares, but it is blind to the opposite colour. So by putting your pawns on the opposite colour, your Bishop can zip around the board without your pawns getting in the way. And, your pawns will control squares that your Bishop can't see, taking away squares from your opponent's Bishop. So your Bishop gets better while theirs gets worse. Mind you, as the game gets closer to the end game, where you may only have a couple pawns left, reversing that rule is a good idea. You want your Bishop to now be able to protect your pawns and you don't want them to become targets for your opponent's Bishop! As the situation changes we must adapt. 
And the last idea, containing a bad piece, is really just a specific example of how sometimes the best move isn't one that attacks or threatens anything, but simple making a move to prevent your opponent from being able to put their piece in a better location. For example, if you were White in the "outpost" example above, and you saw that with two moves Black could plonk a knight there, you might look to see if you can do something to take away the square they have to get to on the first of those two moves. So sometimes the best move is to just restrict your opponent's options, rather than look for something to attack.
Just like spotting tactics in a game, it takes practice to find those types of moves, and to evaluate if they are indeed the best move. You don't want to overlook at good tactic if there is one after all. And especially if that type of move selection isn't something you've focused on all that much, at first it will take a lot of slow thinking it all through. And yes, you will make the wrong choice at times, but that's how we learn to fine tune our decision making.
Anyway, there are a lot of aspects to chess to learn about. Others have mentioned understanding pawn structures, and pawn breaks, and so forth, and all of those are also very important. Like anything, though, pick one aspect to concentrate on most in your games until you feel comfortable with it and think you're reasonably calibrated, and then work on the next thing. You will lose games as you try to use the new information because you'll make errors, but those will diminish, and after a bit your game will have improved to a new level. And, the best part, there will be a whole new dimension about the game that opens up to you and you will appreciate it even more. I sometimes get as much joy from spotting an outpost and figuring out how to get my knight there as I do from spotting a nice tactic.
Sadly, I often fail to take my own advice, and end up blundering, but that too is part of chess.

Got it! Thank you so much for the explanation, I now have a better understanding of how these strategic concepts can improve my game in the long run. I'll definitely put them into practice. Thanks again!

Glad that helped (it really helped me to try and put things into words as well). I've read various chess books over the years, and while things like "outposts" get described, the authors often seem to think the benefits of these sorts of ideas are self-evident (because to them they are). But when one is trying to gain that understanding, it really helps if both the concrete description and the underlying idea of why that is good (or bad). Tactical ideas, like pins and forks, have very concrete benefits, you win material (usually). The more long-term ideas, like "control of the dark/light" squares, or "get your pawns on the opposite colour of your remaining Bishop", need a bit more delving into the reasons. The stuff I've been mentioning are ideas that I've picked up from watching some streamers who include decent instructional comments along the way. Personally, I quite like Gotham as he often will throw in some very good instructional content amongst the trash talk and jokes. I also really like Simon Williams' stuff (the GingerGM), although he doesn't post a lot these days (generally just his Title Tuesday runs, but he's starting to do some more instructional videos centered around a speed run in rapid games; and those are the ones I really learn a lot of the ideas, but it takes playing my own games to learn how to implement them myself. But that's how it works after all. 
I recommend you search around the various streamers and find one that suits you. I would avoid any that focus on traps too much, as traps are things to be aware of, but if you rely upon them you will eventually hit a wall and have to go back and unlearn that reliance and start over again.
Anyway, enjoy. There is so much to this game to explore, and it just gets more fascinating the more you learn.