Hey there!
First of all, let me tell you that what you are feeling many, many people have already went through it - including myself!
People don't understand that bisexuality is as common and normal as homosexuality and heterosexuality (being gay or straight :p). Should you experiment? Many times you won't need to even experiment to know what your sexual orientation really is. If you are attracted to men and women, then you might be bisexual, and since you have a girlfriend but have feelings towards men, then you might be bisexual.
My advice is to not drown in a glass of water. I really don't know your age (pssst) but if you're in your early-mid teens, you have plenty of time. And if you're in your twenties or upper, then you know this is the chance to really get to know you the best. It may not be the best idea to tell your girlfriend about this yet - she might get confused, unloved, ugly, whatnot. Girls overreact, especially if she loves you :) (it is true, isn't it?!). Try talking about this with someone you really trust (your parents may not be the best idea, y'know?) like a sibling (a close sister is by far the best option) or any close female friend (second best option).
Also, LABELS are BAD! Don't try to fit you in the "gay" or "straight" or "bi-curious" or whatever category yet. You've done a great improvement in accepting that you have some homosexual tendencies and you ask for help about it, it is a great step. Now, get to know you. Consider if you really want to be with a woman or a man for the rest of your life. Consider if you only feel "lust" or only "love" --or both-- towards men.
And as a last advice: The best thing you can do is to accept yourself the way you are. There is no "degay-omatic" or "Strisghtning Pills" or even the Facilities that guarantees you that they will make your straight, nuh uh, don't work. The best you can do after finding who you are is working with that and build self confidence and love towards yourself. Be honest, play safe (*coughs* always use a condom and LOTS OF lube if you will experiment *coughs*) and be patient. Answers will come :)
Oh, and I know this was not as detailed as I would love it to be, but send me a PM if you ever want to talk about this, or anything else.
-Jean.