how do you stop this feeling

So  backstory me and my ex broke up a while ago which sent me in a bad mental spiral but I decided to reach out to him to  which probably wasn’t a good idea cause he cheated on me emotionally and implied I was a whore but . Now we’re somewhat friends


Flash forward me and my friend group were hangout at lunch and I look over and I saw him holding hands with one of my best friends I  would normally think nothing of it cause we’re a touchy-feely group of friends but this was different i felt so bitter so angry it make my stomach turn  it also didn’t help that they tagged along when me and him walk to class together they kept holding hands I tried to ignore it but I overheard them say “we’re taking up too much of the hallway and it is not oh ok” he laughed and agreed after that I went to the bathroom and cried I cut myself I laughed I lost it  I remember when I met them first thing they said to me was “I’m gonna steal your boyfriend” now I’m well aware I’m not allowed to feel like this we’re done I can’t get him  back yet anyone that gets too close to him or the thought of anyone else being with him makes me feel sick  I think I’m safe for right now keeping it in cause I can’t tell anyone Else how cause they’ll lash out at me or look at me side ways it’s not my fault that I feel this  feeling so strongly I want it too stop  I’m sure how much more I can take