Middlegames are tough because there are so many things to look for (sometimes very complicated things too), but if you are around 1300s rating, then you likely already know about opening principles and follow them fairly well and now you reach a position where you are fully developed and feel like "now what?"
When I was around 1300s in rating, it sometimes felt like I was just shuffling around and hoping to spot a tactic the opponent doesn't see. We can do better than this. 
What we have to do is create a "plan." This is when the "real chess" starts for many people because this is when you get to feature your own logic, creativity, ideas and so on to this position. One good place to start is to think, "what is my worst piece?" and then try to improve that piece to a more active role in the game. Another good plan might be to identify a weakness from the opponent (like an uncastled King) and then make a plan to exploit their weakness (like opening lines of attack to target this uncastled King). Another idea might be to think what your greatest strength in this position is and then amplify the effects of this. If you have a space advantage, then try to advance and gain even more space; if you have dominance over a certain color-complex, then attack on those squares. This is very Silman-like from IM Jeremy Silman's "imbalances."
The "problem" with the middlegame is that there are so many plans you could have and it is up to you to figure out which you want to come up with in the game. Sometimes more than one plan is winning, so it isn't always just "find the only winning move" but rather, find a way to keep making progress.
How do we know what plan to go with in any given position? It is mostly experience and learning from trial and error. Obviously, the better you understand that chess position, then the more likely you are to find winning plans, but don't stress about it too much because everyone is in the same boat and also has the same middlegame obstacles as you do. I hope my answer helps give you some ideas of potential plans, but the short answer is that the middlegame is where you start implementing your own ideas
How do I get better at midgames? Like every time I play midgame my common sense literally jumps out the window sometimes such that im losing to 1200< as a 1300+ even if they're really bad and I win the games, I'd end off with several mistakes and blunders, not to mention even when my common sense stays, I can't find any forms of an attack, I play the Sicillian ,Slav defense and Ruy Lopez(3 knights if the opponent doesn't play my line), my early openings are good(well good based off chess.com) but I can never ever find any attacks during the midgame, and if i do they're either very risky or plain obvious and easy to defend against, I have to wait till my opponent makes a very obvious blunder in order for me to win, and I lose my early game advantage because of my blunders whenever I do not know what move I ought to make and I do not want to play a waiting game against the opponent as to who takes the initiative. How can I improve such that i don't make too many obvious blunders, and that i can find attacks and tactics because clearly im bad at it. I could probably get to 1400 rating, but i just want to improve rather than up my elo, because there's no point in going past 1400 if im just going to be on the same level as a 1200-1300.