i can't never cum

I'm 19 years old, I have cum once with my ex bf, that was in 2013 now I have my lovely bf we have been together for 1 year in some months, Tbh I have NEVER EVER cum neither him. By the why I do have thrust issues because of my ex it's a long story. Everything has changed since then. when I'm having sex with my bf is feels amazing!!! But... When I feel that I'm bout to cum I can't!!!!:( why???? I just stop nd push him off me. Like I try so hard to cum, he told me that it has never happened to him he NEVER takes long to cum, but with me is different. Why somebody please help me? I do feel a little uncomfortable of my body, nd I have never liked to try sex positions for an ex me riding him... I just now started to try it out but it still doesn't work's for me!