Try playing some lobbies/pugs as each class so you get a good idea of your role as each of them. In pubs, each class doesn't really have a role as much, so a somewhat competitive format could help you solidify on which class you like the role of if you don't know which you prefer the mechanics of.
Or, you could just try playing pocket. You have 300 hp, 4 rockets, and 6 shotguns. The projectiles will help somewhat with demo and the hitscan with scout, so you could still be good at all 3. Conversely, you could play demo and scout, and soldier will keep up as your pipe and scattergun aim improves.
Finally, if you don't think you're good enough for ESEA, you could try playing UGC. It doesn't cost anything so you have nothing to lose.