Most Helpful Guy
You see you problem is you try to not think about him when in all acuality you shoukd acknowledge you guys broke up and just say I don't need him and I'll find better. Literally millions of guys in the world and you're stuck in the one who obviously didn't care about you.
Most Helpful Girl
How recent is this? In the beginning of a breakup, I let myself think of him, think of the relationship, and I allow myself to cry and grieve for the relationship. After I feel like I am done with my grieving and crying, I do my best to not think of him, granted sometimes once in a while something slips and I cry a bit, but generally the worst of the crying has already happened previously.
Definitely go no contact. Continue to stay busy, pick up a new hobby, spend time with friends.
Forgive yourself. It's not your fault he turned out to be a manwhore who didn't treasure you/your feelings. It's okay to have made a mistake, sometimes we love people we shouldn't, that happens. Recognize it is not your fault, there is nothing you could have done to make him less of a manwhore, he made choices and decisions that you could not sway (you can lead a horse to water but you can't make it drink).
And then allow yourself time. You don't magically wake up one morning and you're over a person you cared deeply for. It's a gradual process. Give yourself time and don't be disappointed in yourself that it isn't happening right away.