I cant climax from blowjob….

Hi so im 25 years old and my curent girlfuend love to give head. So yesterday she said to cum in her mouth, i agreed but after 10 minutes of blowjob i was soo close to cumming but then everything fell apart… i told her to stop so we could have sex, but she insisted to give head till i cum. Well she tried for another 10 minutes still nothing. She stoped, and asked me what is she doing wrong… i felt so bad…. I an circumsised from since i was a child so my head of a penis is not so sensible… do you have any ideas how can i cilmax from oral, or any tips…