How to get past a cheating partner

So my boyfriend and I have been together for a few years and I recently found out that he has been cheating on me the entire relationship with his BM. I’ve been trying to move past it and forgive him because I really do love him, but I can’t stop thinking about the things he’s done. I know it’s his child’s mother’s and she’s in the rest of his life, because of their soon to be 18yr old daughter; however the whole situation is still very bothersome. I feel like it taking a heavy toll on my relationship and I just don’t know how to jump out the window again and dive in head first. I’m honestly scared and my brain is at extreme war with my heart. So how do I forgive him or work towards being okay if this is the person I want to be with