the Scoundrels in my guild have no trouble dealing substained dmg. It sounds like you have trouble managing your energy. You know that energy regens faster if you have more. So all out burst is generally a bad idea in team fights. If this is your problem you should consider switching to shadow. But be warned, you'll have a lot more competition for raid spots/loot.

If you want to switch sides because of the story, you should avoid the Trooper. The story isn't all bad, but it seems the author never heard of Star Wars before. The Jedi Consular story on the other hand fits really good. Don't know about the other classes.

Vanguard is a cqc character. If you want ranged dps stick with commando.

You still could choose another server and create a new legacy for your toons.