Liking my best friend....... Help?

I'm 12. My best friends name is Jesse and he's a guy. I'm a girl. I've known him for 2 years. I have liked him ever since I met him, before I thought he was gay.. anyway over the summer he confessed he liked me! :D He asked me out too but me and loving our friendship, I said no. He knows I liked him at that time. So we just ignored us liking each other. I saw him over the summer alot. But when school started we promised we would talk even if we weren't in any classes together. Good thing is we ride the same bus, Morning and afternoon. But turns out he became more umm perverted. Soo I stopped liking him. But we were still besties. Then a girl in MY class, started thinking I liked him, me and her started arguing. I ignored her saying stuff like that. Then one day I went to his house and he was hugging me, and we were talking and laughing, I even learned how to shoot a bow and arrow that day. He spent that whole day not being perverted. So i made up my mind that I liked him again. Not so long after that, I found out that HER nd Jesse were gf nd bf! I was fine with it but at the same time, I think she's a slut and he shouldn't be with him. Just thinking that as a bestfriend... Now I'm thinking it as more than that. It hasn't even been like a week, and she had already started saying "I love you! Forever And Always! ;)." Personally I think there SOO wrong for eachother. So I like my best friend, and he has a girlfriend, what should I do?

No harsh comments, this is my first time going through stuff like this.