Venting. I'm so hurt

Sorry I just need to vent. I gave my all to my now ex for the past 5 years. His kids as well. I been there for half of his kids life. He cheated on me last year and worked past that. I just found out he has reached out to her once again today. Its so hard to walk away. I feel stuck. Idk what to do.

I became his kid's mother and it's hard to let go. I need advice. I know to walk away but how can I do that when I invested my all with him and his kids?