believer

I would have to wonder about the cheating partner - that he is only now shifting into gear when he realizes what he might lose? Is the competitiveness in him that he is doing it? Does he really not want to lose you. However from what you have said, sound like he was not willing to move forward with marriage & kids until the other man came into the picture.
I would have some doubts about this other "wonderful" man as well because if you are unhappy in the other relationship - he might just seem wonderful now comparing him to your current "unhappy" situation. If he really has feelings for you - he may give you your space to sort out your feelings so you can make an educated decision.
I think it is important to spend time on your own & figure out what it is you want before jumping into a new relationship or leaving the old.
it is tough- but only you will know what is right for you.