[M]ight be gay - what about my gf? Advice please!

Hi all,I'm kind of freaking out right now. I [24yo M] came out to myself as bi about a year ago just before I started dating my girlfriend (who knows I'm bi). It's my first real relationship. The other night we were having sex and I couldn't keep it up, I had a few drinks, so that might've been it, but she told me she was worried it's because I'd rather be with a guy. I've had a huge amount of anxiety since then because I realized I really don't know if she's right. I fooled myself into thinking I was straight for years and years, maybe bi is just my safety bubble. I love my girlfriend, I think she's an amazing person, but lately I've been thinking maybe the love I feel for her is more a "best friend" kind of love not an "in love" kind of love. I really don't know, and I also don't know how else to find out other than breaking it off. The problem is we're planning on moving away together in a few months and I'd still like to go with her (not to mention we're financially dependent on one another when we do move) even if we are broken up, but I don't know that she'll feel the same way. We've talked about being friends if we break up and both agreed that we want to, but I know it's naive to say that while in a relationship. I hear it's possible to be friends with your ex, but moving in together after only three or four months? Is that even possible? Do you think it will it help or hurt I tell her I'm not sure if I'm gay? Anyone have a similar experience?Relevant information:I've had sex with guys and prefer it to sex with women (not to say I don't enjoy sex with women, but it's def not as good).