I have been stuck in 1200 for years, what should I improve more

I don't know if it's because my opponents are getting improved or they're cheating or what, but should I improve more ?

Play games and review the games either on chess.com or Lichess whether you won or lost, it can help fix any mistakes you made. Play puzzles, do endgames and tactics.

It's been done, but it's still stuck

Then... sticking to one opening first and then getting good at that and then branching out to more. Watching videos online(?)

okay

I got to 1000 pretty quick but then it took me several years to get to 1300. I think I’m maxing out lol

Hi 
I already understand the principles of opening: control the center, create connections between rooks, put knights at the outpost. However still, I often make blunders because I find myself in an disadvantage position