Letting Go After 4 Years

Has anybody had to let go of somebody that they spent years with and a that included a lot of your time? I didn't think I could do it but I blocked his number and despise him more every day. There has been other women this whole time including his baby mama (their child was born before he met me) and they claimed they had nothing going on. I'm a bit hurt but I'm making it day by day...Any tips on not losing the fight to wanting to just lash out at him?