he cheated on me-how or should we start over?

We arent married but live together in a new home. I have one child shes seven, and he and I have his first child -a boy- due in october.
My issue is, I found out in november that he was seeing his ex behind my back for the first year we were together up until about a year ago. I know he loves me and doesnt want her, or see her now, but I am pretty sure they still talk behind my back,,,he had two seperate email accounts I found with just her address in them…And she wont give up…she keeps telling our friends and him that hes not happy and I dont know if I should give him another chance..I think even though it will be really hard financially, that I should move out and get an apartment…if I do this I know we will be over for good-hes really stubborn. What do you think? This baby was something he always said he wanted, but he also said he wants to marry me–but uses money as an excuse not to…that really bothers me.