Hey, i know how you feel. I quite often used to think i'd really like to be a guy. still do sometimes. Its nothing to particularly worry about, its most probably a subconsious thing. Especially if you have low self confidence or maybe arent the strongest person around if you catch my drift. The guys you see always have loads of confidence and are strong, and it can become very easy to say "if i was a guy that's how i would be" though i may simply be being media minded, lol! (the theory of we see things that we want to be and use the product, the gender in this case, to become that).
As for your sexuality, if you've always had an attraction to both, you probably always will. Its not a problem, and im told women are the better kissers. You get more choice. I really wouldnt advise cheating on your bf, i mean, it would make you as bad as he is really. Dont mean to sound harsh if i do, but you need to remember how crushed it made you feel! By all means, experiment, i advise it whole heartedly. You'll get to know your own feeling so much stronger that way, if its not for you, you'll know it isnt!
I hope this helps. pm me if you want to talk, i often lose track of topics!