She might truly be a nice girl, but whether she realizes it or not, she still has feelings for her ex, otherwise she wouldn't even entertain any of his attempts at contacting her. If she REALLY wanted to make a go of it with you, she would have no time for him at all. You need to think about yourself before you get really hurt by her.
I know others who have been through this situation before and, like them, you will probably carry on seeing her because the heart wants what it wants after all!
You'll just have to learn the hard way if you continue to see her, with him still very much in the picture.