Half in love with two people...

So I've been in a relationship with my best friend for nearly two years now, and we'd sort of been unofficially together for about a year previous to that. We're both girls, by the way. We have had a few arguments but none recently, and none face to face - all over texts and messages. I feel that I love her and she makes me happy a lot of the time, but at the same time I know she's not right for me. We don't see each other in person very often at all, I don't feel confident around her and I feel like parts of our relationship are too fragile to work too much longer. The thing is, I feel like we rely on each other's contact to remain stable. I have quite a lot of mental issues and talking to her can often help with that; she has difficulty with friends and her family situation isn't the best. We're literally texting all the time! I feel like if I broke up with her it would ruin our friendship which I value a lot, and I'd lose contact with a lot of people. Then there's the other girl: my best friend in school, who I've known for about a year less than my girlfriend, but I'm very close to. I've developed a weirdly intense crush on her in the past few months, and now I'm fantasising about her daily. Our communication outside of school has become much more regular, and it's obvious we've gotten a lot closer. A few of our friends say we'd be a great couple, and while we joke about it occasionally it makes me feel weird because I know that I actually like her in that way. I don't know if I can imagine myself in a relationship with her, though, and I'm pretty sure she doesn't like me in that way - although I don't know how to tell, since she's quite private about crushes and stuff like that, even on celebrities! I know she's bisexual, but that doesn't really make much of a difference. I love her, but I don't know where it's meant to go from here. I don't know how much longer I can deal with this. I can either stay with my girlfriend until it falls apart and I have a breakdown, I can tell my friend my feelings and get rejected therefore losing my best friend and feeling guilty, or just break up with my girlfriend and give up on both of them. I hate this so much.