I completely understand your pain. My ex left when our daughter was 6 months old. He left the night the bomb was dropped. Since he had a plan to go live with his sister THAT NIGHT, I instead for 2 months went to live with my dad and stepmom. My ex never looked back, was fully involved with OW and they have been married for 6 years now and that woman is my daughters stepmom.
For year I struggled with that part. Not my ex so much, but the time taken away from me and my D. The splitting of the holidays from the very beginning. I hated them both for that. We did IVF, she was sort of our miracle baby, then I had to split my time? I am luck in the fact he did not want 50/50. I also told him over my dead body:) (I was an awful DB'er).
My point being, everyone makes mistakes in our marriages. I beat myself up the way you are. I own my part only. My ex was not good to me since the day I met him, but I always put him on a pedestal. My mistake? I finally couldn't take the abuse. Hormones during IVF finally had me needing him to be decent to me and put me somewhere on his list at the top which I never was. When he didn't I became b!tchy. I became defensive, with self protection, because I always knew what was coming. I mentally prepared myself.
I realize, after 9 years on these boards, there is no strategy, there is no really changing game plans to get them back. As long as your original plan was to simply be the best you that you can be and that makes you happy with who you are. And what is meant to follow after that is what will follow. It takes two to save an M but only one to be who you want to be.
Fast forward to 9 years later. I have had so much growth and I am happy with the person I am. I was always afraid to look in, but I no longer am. I take it overboard sometimes beating myself up. I am civil with my ex and his OWW. All I care about is my daughter is happy and healthy.
I do hope for you that perhaps your ex decides to make changes for himself and for his family. But right now, you protect yourself legally, and just be true to yourself.
