I'm hurt and confused *trigger warning*

Hi. My name is heather. I guess I'm on here right now because im scared and confused. To start from the beginning. I'm in my 3rd year of college right now, for the last 2 years I've had this roomate, liz. She became my best friend, and I kinda developed a crush on her. But she had a crush, or so I thought on another girl. But then she told me that she actually liked me. But this other girl was trying to get into liz's pants. And she came to me crying that I stole liz from her. At the time, I wasnt sure if liz and I were going to be commited or not, and I didnt want to lose my friends, so I let this girl have sex with me. I didnt want it, but I thought that was what liz wanted. And it hurt, and I tried to say no but I didnt. Afterwards this girl tried to tell me I needed a man and I didnt really love liz, and it hurt. But now me and liz are in a comment relationship. I truly believe she is my soul mate. However. when I think back to that stupid night, it scares me, just the thought of being around that girl terrifies me. And the worst part is I know it's my fault. Now I found out this semester she is going to be in one of my classes. And I'm scared. I don't know what to call this and I don't want to be around her. I feel so dirty and used. I just dont know.