Help!! I really need advice, what do you think I should do?

consider the fact that he will forever be part of her life because of the child - unless he is willing to relinquish his parental rights. that's a lot to deal with. are you willing to have that ALWAYS be an issue?

Number 1: Until her marital situation is resolved, don't act like husband and wife with her. She is still his, and if he wants an excuse to take the child, or get alimony from her, why hand him the evidence?
Second: Giver him a chance to earn her affection back, from your perspective, to see if he is willing to make the investment. Then he loses any leverage he may have now, using the excuse that you stole her from him.
Third: If you invest time with her, make it in public places, where people can see how you act and gain respect for you.
Fourth: Pray. Normally I would put this first but, in your case, I will suggest you pray as you do the first three, and then pray more. Let her know what you are doing and see if she is willing to be patient and wait for things. Delayed gratification is always worthy of the investment.