I'm a level 31 Guardian Fighter. If you want to keep playing the GF then (as someone suggested earlier) try to spec for more damage. Take all offensive stats on gear and take offensive powers and feats. When I realised this, levelling became infinitely better for me.
I'm pretty sure you can do this. On Gateway its a little square in top right of slot. Hover your mouse and it says cancel. Not sure if it's the same ingame.