I'll share something personal with you - when I started tf2 I played medic, pyro and engineer pretty much exclusively for the same reason that you did. I couldn't jump, I was running in straight lines as scout.
I was very much embarrassed to even try, and when I did I got flustered fast and switched to the classes I could contribute at least something with to the team.
Now, a person with more self confidence or one that doesn't have a weird hang up about himself (aka not me) would probably go to a jump map and just grind it, play MGE, keep playing the class until he got better. I wish that was me. It wasn't. I only play a little bit after work and didn't want it to be a frustrating and negative experience. I just want to play and enjoy the game that I enjoy, y'know?
Eventually, I got fed up with myself and this is how I sorta solved it: I made an alt. Told no one. I don't know WHY it helped, but pretending it somehow doesn't count if that bad f2p stereotype is not me but this alt thing helped. Not playing on my main account took the pressure off somehow.
Joined my preferred mode that isn't shit (5 cp or koth or payload or uh, Egypt), clicked on the random button. A heavy. Great. I am playing a heavy for the entire round. A demo. well shit. I am now stuck being a demo for a whole round.
Through doing this I discovered that I am a pretty good sniper and got ok enough at scout that he is my most played and best class.
I just now started to be sorta decent at jumping after a pretty embarrassing amount of practice. I had to find an approach I liked and wasn't the depressing and boring experience that was the jump maps for me.
Using an alt to practice classes I was completely rubbish at boosted my confidence in practicing them. NO ONE has to know it took me a fuckmillion hours more to get sorta-kinda-acceptable at jumping or to perform at a level someone with just 50 hours at a class does. Once I got over that, I started to enjoy the game more and play those classes on my main account. Even if I am not that great. It gets better, it really does.
If you find no fun in improving at a class because he's boring to you, say "fuck this, this isn't fun for me. I am a sentry main and a videogame is not my job for me to grind so much for it". But if you want to be one of those people who can play soldier and think you'll genuinely enjoy being that person who can and DID, be honest with yourself and figure out how to "fool yourself" into having confidence.
Besides, getting practice as soldier or demo does help you as medic and engineer and heavy. you see it from their point of view. you'll have easier time following the soldier rollout when you use a quick fix, perhaps evening landing a market gardening with your ubersaw. Suddenly you'll see that you get rocket jumping and can use it to get away when a soldier bombs you. you'll know what sort of behaviour makes it hard for a demoman to take down your sentry, etc.