my real problem was cumming on the way in. If I had not fucked for a while, which was mostly the case, my cock would be rock hard and so sensitive that as soon as the head got in and started sliding into a cunt, I was cumming. Generally, I just kept fucking and in a short while would cum again.

I've never had a problem cumming while masturbating. I can make my self cum in less than a minute if I need too. But fortunately I can last while fucking as long as I want too.

yes, I had this problem with a particular woman......It's a psychological probelm where, however much you want to have sex, your sub-concious mind knows that the woman is the wrong one to be in bed with and the outcome will be bad for you...your sub-concious picks up on very slight give aways...perhaps she is hoping to get pregnant or use the coupling against you later...perhaps she is a potential blackmailer.....perhaps she is looking for a husband....perhaps she will not be discrete and tell another woman that you actually like better.....there's lots of reasons

You're too young to have problems. You should ask a doctor. However, having said that, there are many women who can't cum during penetration. Men too, but it's rarer. Actually, I think it's a matter of relaxing. Don't think you should cum during Penetration. Just fuck, pull out , and use your hand. I often do that because I like yoo watch my cumload cover her pussy, boobs or face.
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Also, when I use a condom, I don't like cunning in the condom, not that I can't, but I don't like it. So, I'll fuck and then pull out, get rid of the condom and cum freely by either wanking or being wanked by her.
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The less you think of it as a problem, the easier you'll be able to cum. Sex resides in the brain, not in the genitals.

I have had that problem when I was close and stopped. Like if she giving a bj and I was really close then we switch positions. Sometimes it's really hard.