Should I not put all my heart?

I am 17 and Bi. I met this girl Nicole and we just started to actually hang out about 5 days ago. The first night together we ended up drinking and we hooked up. She did more to me than I did to her. She didn't want me to because she said she thought she was prego. Anyways we hung out another night after that and we drank again. Well ever since then I can't stop thinking about her. She's back with her boyfriend who she keeps on going on and off with. I don't like the guy either cuz he hits her and treats her like ****. If I could I would beat the shiz out of him, but I don't have the strength. Then when they break up she goes around and drinks and hooks up with anyone. Idk what to do. I like her a whole lot and I want to tell her how I feel but how would I even go about that?? like I just want whats best for her and I can be that. Ik she's scared to fall in love with a girl but so am I?? I just need advice from someone. Like what do I do? I can't just let it be. I can't stop thinking of her and that cute smile. Shes perfect inside and out just not her choices so much. She's gorgeous and shes always making me smile and laugh. Shes amazing. I can't just let her goo. What do I doo??