Ok so here is the problem. Say I'm with someone. We're really really good together and I can't bear the thought of hurting her. But then there's this other girl who every time I see makes me feel a bit like I'm in free fall. The problem is I don't even know if this other girl feels the same way, but we seem to have some really amazing chemistry.

So my predicament is as follows. I have two options:

1: I leave my girlfriend. If I did this and I got the other girl, then it could be worth it. However there are two other outcomes that could be really bad. I could leave my girlfriend and it not work out with the other girl, that would suck. I could also end up with the other girl but then realise that I don't want to be with her after all.

2: I stay with my girlfriend. This could turn out well. However it is also possible that I could end up regretting missing my opportunity with the other girl and our relationship would fail anyway or I would be really unhappy.

So yeah those are my two options, both suck, but still. Advice?

Oh and on the topic - a lot of people seem to be sure that loving two people is impossible. Perhaps. But expecting someone to 'look into their heart' and find which one they love most is far from an easy task and one which I have tried for a long time and so far failed to do.
Finally a little background. The other girl knows I'm taken, my girlfriend does not know my feelings for the other girl however.