Take cf yourself, it is a short and simple statement but the only thing you need to do. Every time you allow him in your heart again you take care of him and not yourself.
My ex husband cheated on me and when I found out I left and he was with her for a year. However I did not know that and continued to try to work things out with him, as he claimed to be desperate to do so. When I found out the real truth, I was spurred by anger and by self love. I called her and told her, called his mom and best friend whohe had also been lying to, and moved across the country to finally end things for good. Dramatic, I know, but I still feel empowered by my self care and love 4 years later!
Take care!