What training methods do you suggest so I can improve in Blitz?

I'm currently a 1342 Blitz and wanna raise my play up to 1800 maximum or 1600 least by the end of 2021.

Currently, I just play games and that's all. I do puzzles sometimes not always and that's about it. 

So, if you guys have a way to help improve then do so.

Note: My opening repertoire is wide. So I know I'm perfectly fine there. I just need to improve my middle game and work some more on my endgame.

focus on 1 opening. When you're at a lower level, opening doesn't matter since when you gt to mid game with favorable positions you won't know what to do with them. Focus and hit hard with 1

You have 2 options:

1 - study and get better and that improvement is going to reflect in your blitz rating.

2 - play bullet. You won't get better at chess, but you might get some 100 elo points better in blitz..... also keep in mind bullet is not chess

I recommend #1