Cannot get an erection whe I need to, only when I don't???

Hi. I am a male 21 years old and would like some advise.
Since about a week ago, I got a new girlfriend. My problem is that when we are together and we start kissing and hugging I get an erection. Usually I stay with that erection for quite some time and after that, and when I check my underwear, they are wet (Looks like I ejaculated on them). Even after kissing for 1-2 minutes, I get an erection. However, when we were about to have sex and started kissing and got naked, it was impossible for me to get an erection. I am really worried about this, as I never had the same problem in the past. We only tried to have sex once and this happened. I have read somewhere that stress, alcohol and other factors may be some of the reasons for not being able to have an erection. However I am not stressed, I am athletic and do not drink a lot of alcohol. I was wondering if the reason why I cannot not get an erection when I need to is because I get an early erection when we start kissing and hugging and as a result my penis is “tired” or even “finished” a long time ago, when we started kissing and cannot get erected anymore. We leave very close and see each other very very often, as a result, my penis gets erected for quite a long time. 3 hours a day or more sometimes.
I would really appreciate some advice. Thanks