What do I do?

So there’s this girl I really like at school. I havent said anything about me liking her bc I have a “boyfriend”. She’s flirted with me before and everytime she sees me with him she would look at me up and down and give a smirk. I did tell one girl in my class bc she’s lesbian so I asked her how she knew she actually liked girls. When I told her about my crush her smile went down and told me that was her ex... she told me this girl was a cheater and a liar and always made her feel like shut in the relationship; she also told me that before the Christmas break the other girl stood on top of a chair and sang “all I want for Christmas is you” and told me she was bad news. But then a few days later the girl I have a crush on came into the girls bathroom and started flirting with me and she said I could pass off as a freshman so I called out the fact that she is a sophomore and I’m a junior and she said “no I’m a senior yuh heard! Bouta graduate 2019 so stay in your lane” and I said “mhm ‘Senior’ not from what ive heard sophy” and she came close to me and put her hand against the wall behind me and said “so you’ve heard about me?” And she licked her lips and looked at me up and down. My bestfriend was with me and she said I was really red. But I feel guilty bc this girl is my friends ex and I could tell she still has feelings for her and cuz I have a boyfriend😭 I’m so confused on what to do