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

Having an erection when you hug and kiss is normal and in itself should not be a problem. If, however, you have what is called "premature ejaculation", then you might try a low doze of an SSRI-drug. They make it more difficult to ejaculate. I cannot promise that this will help you, just that it might help you, and may be worth trying if you cannot accept it the way it is.

Of course, you should not try any drug without discussing it with a doctor, in the case of SSRI-drugs preferably a psychiatrist, as they know most about psychiatric drugs.