Should i leave or stay?

Ive posted some time ago about my husband not finding any faults in his cheating ex wife. How he still have pics of her alone and them together. Well, i just found a message he wrote to her the day she left him, which was 7yrs ago, that in the next lifetime he will find her, and wont allow anyone to steal him away from her. He also said that it was his fault she left, he will let her go, but hed never let her go in his heart. Idk anymore... this makes me feel so degrading and so sad. I feel like a rebound. And its like i have his body, but not his heart. I feel he doesnt truly love or want me, im just there to give him kids, give him a life, to look good to ppl, and so he wont be lonely. But ill always be second best to his ex wife, ill never have a place in his heart. Yeah, i replaced his ex wife physically, but not mentally. I regret it so much, i never thought it would come to this when i found out he was a divorced. I thought maybe having me and our kids will change the way he felt towards his ex and whatever happened to them, but i was wrong. Idk anymore... its mentally destroying me. Ive talked to him about it, he just said im his #1, but it doesnt mean anything to me. Especially w the things hes done and said about her and i. He had compared me to her back then and said she was better. He said hes never had this much stress w anyone, but me. Should i stay and help him thru this or leave?