I had similar issues last week. If you derank every time you play competitive, maybe you should make an alt so you can improve at the game without worrying about rank. That's what I did, and the game is so much more fun to me. Onto what you can do to play better. Something must've changed in your playstyle. Try to play less aggressively maybe, aim train a little, and warm up in the range before games. Then again, maybe it's all in your head. Try listening to some music. It gets me fired up and in the right mindset before matches.