In love with pregnant best friend

I guess I've never really known what sexuality I have. I've never been attracted to anyone..ever. I'm a lesbian..I guess. I'm not even sure if I am..I guess I haven't really figured everything out yet.

I'm 24 years of age, and I've been with guys before, but never felt anything for any of them but I've never been attracted to girls either. I thought maybe I was just one of those people who were asexual.

But suddenly, out of no where, like I was hit by a bus, I started having feelings for one of my best friends. She's already one child from a preview relationship and at the time, was not with anyone.
We were at a party at her house and we both had said we'd never kissed a girl before, so everyone was like kiss, kiss..and we did, and my heart nearly exploded.

Any other time I'd kissed guys, it was very much Bruno Mars..had her eyes wide open..why were they open. Nothing. No spark, later on in the evening she reached behind me for something, brushed my back and again, my heart just started pounding..I'd never felt anything like it.
Everyone had left and she wanted me to stay, we were both very drunk but she wanted me to stay, and I felt like she wanted more.. I was afraid at the time, but I wish I had now, more than anything I wish I'd stayed.
Now she's in a serious relationship, is pregnant and engaged, anytime I see her I have to do everything that's possible not to lean over and kiss her. I thought maybe now she's pregnant those feelings would go, but the bigger she gets the more attracted I get to her.

I just don't know what to do. She's the only person I've ever been attracted to. I can't tell her, it will ruin everything, but I can't bare to spend time with her, but the thought of not seeing her kills me.