This happens to me aswell. Since I started playing I have improoved a lot in things like needle or trap, but not avoidance. It gets kinda frustrating, I can't complete any fangame with bosses. Not even CTBG :c
I thaught playing other games such as Geometry Dash or Sound Dodger would help me improove but I still suck at it ;-;
Dodging in these games is inherently different from dodging stuff in fangames. At the end of the day, fangames are really their own genre and the best way to improve at bosses in fangames is to play bosses in fangames. Everyone has given great advice.