This is an interesting topic. i have a few friends who can’t get off from a girl giving them head. out of all the girls in my life I have gotten head from (around 15 maybe?) only 2 have not made me cum. One was due to too much teeth, the other from poor technique. I wonder if some guys just cannot ever get off from blowjobs, no matter how good it is? I think as you get older and more experienced you might get used to the feeling. Are you new to sex or blowjobs? I know when I first started getting them in high school it didn’t take anything special to get me off. If you’re new to it you might be getting used to the feeling. If the girl does it well, she might be providing you with more forms of stimulus than sex does. If she is sucking you, stroking with her hand and rubbing your balls, that’s three different forms of stimulation. Sex provides warmth and friction, but so does head. I think you may just not be used to all the stimulation a good blowjob can provide.
So 1. Congrats, your girl sounds like she gives good head.
2. I have tried to build stamina in blowjobs, as practice for sex, and especially when it’s foreplay before sex. Doing kegels, breath control, reverse kegels, etc have all helped my stamina in holding back during a blowjob.