I cant get over my ex..still

I cannot get over my ex. We have been broken up for about three months to the day and I can't stop thinking about him. I caught him texting two of his ex's behind my back to hangout which a part of the reason we split. Also I came out while dating him. Idk what it is but I literally have never been in this situation before. We have tried to mend it but we just argue so much that I said for him to move on b.c it just isn't fair to us. But I am still stuck on him. Anyone have any insight.