Why do I keep thinking about my guy friend when I’m dating this incredible man I’m seeing?

I’ve been seeing this guy I met at college for three months and I really do like him. He’s funny, smart, honest and he’s so cute. Love that he does not take shit from anyone.

I really like him but lately I keep thinking about my guy friend. I’ve known him since elementary school so since kindergarten. At first I got annoyed with him following me EVERYWHERE and he ended up making me laugh and became close friends. When we got older he started asking me out to the school dances which I rejected, and prom twice. I ask him why doesn’t he ask any other girl. I rejected him when he asked me out a lot. I went with him once as a friend in junior prom because the guys were taken. The second prom I went with my crush which ended up in a disaster. He humiliated me for being that fat girl. I don’t understand why I keep thinking about my guy friend lately. Like I keep fighting it and my mind keeps saying his name and not my date. My heart pounds when my guy friend comes near me. I don’t understand. I don’t want him. My best girl friend told me I REALLY need to think about this because I’m gonna end up hurting my date. I’m starting to develop feelings for my guy friend and she thinks it’s because he’s improving himself, maturing taking care of his health or whatever.
I wish my uncle hadn’t asked him if he had a girlfriend last year, He in my family thought that we were dating but I keep telling them that it was just a joking we’re just friends. He likes my boyfriend, but he thinks I’m making a big mistake. My uncle also said having someone that understands me, can relate (we’re both autistic) and he loves me so much. I love him too but as a brother.