hello, i will tell you what i did to improve
go out of your way to watch and analyze youtube videos or demos from good jumpers. look for world records, top times, speedruns, and study what each jumper does in this run. you can find these videos on channels like TF2RJWeekly and TopTime, but also on smaller individual channels like gebobus's channel or xhappy's channel. check our "jump videos" section on this forum for plenty of content.
try to recreate the motions and strategies they used in their videos. this will not be easy at first but after a while of practice you will absorb a lot of knowledge. it's hard to improve without the catalyst, and studying videos will help with this. as a side note, if you find a server with a really good jumper in it, watch them and see what they do. if they're willing (high level jumpers usually are) ask them to explain to you what they did and how. remember, some people may pick up a skill inherently faster than others but everybody learns at a different pace. slower learning does not mean you are not made for rocket jumping, so keep at it and see what happens.