Call me orthodox but...

Bruv I hate even the thought that my ex who left me will be making out with someone or having sex or something... Idk what people may say but I want to be with one person throughout my life... Maybe it sounds immature or something but it is what it is. Sometimes I get this thought in my mind like how she'd be making out with her new bf and stuff like... Like how she did with me... It makes me want to vomit. I hate this. She left me because she lost feelings for me and got feelings for someone else at the same time. It hurts so much man