paranoid

Boyfriend has a sister and she brings around all these girls I don't like and I don't want them trying to talk to my bf. A girl he used to like still hangs out with his sister. She has nothing on me but I saw how they used to talk to each other he was in like 8th grade. I always bring up the past and can't let things go. Me and him have been together for 3 yrs and I recently found out they had a thing and I don't like it. I just want to move forward. And not be parodied. He says he wants me and only me but I forget a lot. Any ways you can help me. I'm 18 he's 19 I'm old enough to know I shouldn't let that get to me but it does and I can't help it.