thinking about my ex (venting and in need of advice)

i have been thinking about my ex boyfriend in a non romantic way since we broke up 2 months ago. i have no romantic feelings for him whatsoever, we ended on bad terms and i have a lot of hatred towards him because he not only went behind my back, he went behind all of my friends backs and even my now boyfriends back. when i think about him, it is all about karma coming to him and i feel bad for thinking about him at all because i want to forget him, but my brain is not letting me calm down with these feelings of hate. everything i see him, i want to yell at him and tell him about how much of an ass he is, but i have to control myself. context of our relationship, this was a high school relationship (which i am still in), he was my first major boyfriend, my first kiss, and a first for a lot of physical stuff. i also went on my first date with him. we dated for like 10 months, so i am not sure if this is a normal thing that people feel after their first serious relationship, or if it isn't normal. i do feel like in love, i am a little obsessive and very romantic, for example, i get my partners a lot of gifts, i write long letters about my love for them, and i think about them all the time, so i am not sure if it is a result of all the obsession being turned into an obsession of hating. i saw this thing that your brain has a hard time forgetting someone that you have been with for a while, so i am not sure if it is that. does anyone else have this problem or advice to help me stop thinking about him?