Two tips that are the most important at your level, by far:
1. If you can take one of the pieces of your opponent for free, take it.
2. Don't hang your own pieces. ("Hang" means put it where it can be captured for free.)
One bonus tip: Play a longer time control where you can think of your moves. Right now you are playing 3-minute blitz, which is way too fast, and as a result every second move is a blunder.
I have really been struggling with developing my pieces and forming a strong middle game. What are some tips for entering the middle game or dealing with it?