He hasnt let her go

My husband had an ex wife, who he married for 5yrs, and dated 3yrs. Highschool sweethearts. She divorced him in 2011, because she found someone new. He began dating again, a yr later. Then we met 2013, dated 2014, a yr and a half after his ex fiance ended it. Anyways, its been 7yrs since his divorce, but hes still holding onto his ex wife. He hasnt let her go yet after all these yrs and new life beginnings. Idk anymore. None of them have kids w him, well his ex fiance did, but she aborted it. We have 2 boys, been together for almost 5yrs now. But nothing new in this life of his, is changing the way he feels about his ex wife, and it cant change his feelings to let his ex go. If i knew this, i wouldnt marry him, but he lied to me. I feel like i am just a rebound. What am i to do? Should i leave or stay? He always say to me that he lost her. If only he didnt do this or that, she wouldnt leave him. If he was happy w me and the kids, he would feel thankful she left him. Cause if she didnt, he wouldnt be married to me, and our kids wouldnt exist.