My VERY HOT girlfriend can't get me to ejaculate?

I'm 20 years old and I've been going out with this very hot maxim model (really happy and proud about it) and we seem to love each other, but it seems like she cant get me to ejaculate during sex...so i last about 2 in a half to 3 hours then i try to use my own hand to finish. Sometimes i may Ejac during sex. But the main problem is when she gives me Bj's..she does it so good on me but yet I cant FINISH!!! Last night it seemed like she was getting frustrated by it and just walked away. I felt like crap... do you think this will effect our relationship? Do you think this is a big problem?