I can't ejaculate when stimulating myself and I cannot enjoy when I'm with my girlfriend

Hi, I'm 18!and have gone through the puberty, since I was like 13 I masterbated by rubbing my penis against something soft and I can cum. However whenever I try the traditional method I don't cum and sometimes I think piss comes out which is disgusting. The issue is I now have a girl and I can't enjoy our time together because I'm scared of things going wrong and she thinks I don't enjoy it when we do stuff as I don't cum. Do I have a problem, or do I just need to keep trying to wank normally? Any help is greatly appreciated,