Hello, I'm new to these forums, but I desperately need advice, I am in a weird scenario and I don't know what to do.

Some Information on myself first:
I'm a Male
18 y/o
virgin

about 2 months ago I began dating this girl and I haven't dated anyone for over a year before that. She is not a virgin. recently we became quite serious, and almost inseparable at times. I really like her a lot and would like to lose my virginity to her. but she spent 2 months teasing me up to the point of coitus and then backing out. then today after an hour of foreplay for which (I had a stable erection for the majority of) she finally told me to get a condom. In the process of finding a condom I lost my erection and was unable to get it back up.

Although she was not upset with me and even very accepting of the fact that I got nervous I still felt very inadequate. after a while of cuddling we began fooling around again and i was able to get it up partially, but not for long before it was limp again. I don't know what to think, and this is really bugging me so any advice is greatly appreciated.

P.S. If it means anything I haven't masturbated since about a two weeks before I met her. and recently I don't feel the sexual drive to masturbate and I no longer wake up with erections in the morning very often.

I just keep wondering what the hell is wrong with me. I have been trying to convince her to take the next step with me for a couple weeks now, and then she finally does and I can't do it.