I struggle with middlegames, please help me!!!

I am working on the endgames with a chess coach and I have a few openings down. However, knowing what to do in the middlegame (when material is equal) is hard for me. Any tips? 

(I also struggle with opposite side castling.)

Generally, you have to understand the basic endgames before it makes sense to work on middlegames.

For example, there's no point learning about pawn weaknesses and isolated pawns - if you don't know the endgames surrounding converting or holding draws with or without the extra pawn (eg. rook + pawn vs rook). Or even worse... just simply blunder your pawn advantage away. Often knowing your endgames will steer or drive your middlegames because you know what destination you're trying to head toward (ie. simplification, or opposite coloured bishop endgame, etc).

At your rating level, I'd honestly say that endgames and tactics will be the best use of your time until you're at least 1300 before I'd start shifting focus to middlegames.

The pay-off for just continuing to level up on Endgames and Tactics is just too great at this stage IMHO.

As for a basic middle game book - How to Reassess Your Chess by Silman is a good intro to the subject. Would recommend it. It's easy to work through.

I looked at a number of your games, and it seems the endgame coaching is paying off. As St4ffordGambit says knowing the endgames that will result from advantages you have gained or weaknesses you have is extremely important. Tactics will still continue to be important to focus on. When it comes to the middle game, and understanding what to do that will come down to a few things. The first thing is learn how to read a board, and understand what your advantages and disadvantages are and for your opponent as well. This is most important, and will help you in any position. The second is from your opening choice. Openings aren't that important at the 1000 level, but having a chosen opening that you consistently play is the most important thing. You will need three, and that is it. One to play as white, one to play against e4 and one to play against d4 (of course there will some specific openings that you won't be able to play said opening against (think Scandinavian, Caro Khan, etc.), but you can learn how to play against those through the principles, tactics, and post game analysis). Don't worry about Nf3 and c4 yet. They are rarely played and often transpose into one of the d4 or e5 openings. This is something I did notice it seems you play multiple openings as white, and don't play a consistent opening against e4 from the games I looked at. While you can get away with this, playing a consistent set of openings will help you learn what the middle games plans are for your chosen opening. The specific move order doesn't matter as much right now.

A good book if you want to get one is The Amateur's Mind by Silman.

Here are some resources to help with the middle game.

Chess Vibes Strategy and Tactics Playlist

Chess Vibes Book Study Playlist

Good and Bad Pieces Article

Strong and Weak Pawns Article

Planning Article

Evaluating a Position

Improving Your Chess - Resources for Beginners and Beyond.....

https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond

@1

"I am working on the endgames" ++ Great

"I have a few openings down"
++ 'just forget about the openings and spend all that time on the endings' - Capablanca

"knowing what to do in the middlegame (when material is equal) is hard for me"

++ 'Tactics is knowing what to do when there is something to do.
Strategy is knowing what to do when there is nothing to do.' - Tartakower.

Generally you should move your piece that stands worst.

Study of endgames will help you decide what to do in the middlegame when material is equal.
From endgame knowledge you will know which trades to aim for and which to avoid,
which pawn moves to aim for and which to avoid.

'the endings can be studied and mastered by themselves,
the middle game and opening must be studied in relation to the end game' - Capablanca

"I also struggle with opposite side castling"
++ Opposite side castling can lead to tense, sharp games needing much calculation.
Key is to control the center: a piece in the center can attack and defend at the same time,
and if you control the center your opponent has difficulty bringing his pieces to attack your king and to defend his king.

Here is a famous opposite side castling game for you to study.
https://www.chessgames.com/perl/chessgame?gid=1044020