BF is cheating with his ex wife... And I'm pregnant

My boyfriend and I are expecting our baby girl in June. He still talks to his exwife daily, and goes much out of his way for her. They have a 3 ye old daughter together, whom I love very dearly. I consider her my own.
He speaks to her in too many intimate ways and she reciprocates the same. He has even told her he still loves her and always will. I've had many conversations with him about this asking him to not continue that relationship ship with her if he was wanting to really be with me. However, he won't and says he can't. I know it's because he's not over her and can't move on, but he says it's not. I don't know how to leave, and I know that I should. :( My heart is broken into a million pieces and I can't take any more heart ache. 

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