Can get erection but not during sex

Hey guys, Im 17 and I was circumcised 6 month ago.

Me and my gf started having sex about 2 month ago and its not what I expected it would be.

Basiclly, when she gives me a blogjob, there is no chance in hell that I would lose my erection but when we start having sex I can easily lose it.

Now, I don't know why or how, but one thing is for sure, It is not physical. I can masturebate easily, and get a blowjob and never lose erection even if she did it 7 times in a row.

Usually, I dont have problems in the first sex, but the first sex is not so great because I sometimes cum before she reaches an orgazm. So we do it again, and the second time is sometimes okay but also, I insert my penis and I lose erection in a second.

I think its because I can't feel anything. After a BJ or the first sex my penis is "numb" I can't feel a thing and thats why I can't maintain an erection. I think.

The weirdest thing happend to me the other day. My parents were gone and I had the house for myself so I invited my gf to come. We had some fun, started having sex and everything is okay, after a shower together she decided she is turned on so while watching TV she started touching and licking my penis. After about a minute we went to the bedroom and it was amazing! just AMAZING! She reached about 3-4 orgazms so after about an hour we did it again, and so on...

Eventually, we did it 4 times in about 4-5 hours and each sex she reached 4-6 orgrazms before I even reached one.

Whats funny about it is that when we woke up, I tried to re-create that night, but with no success. I couldn't even keep it up for the first sex. I dont know why. I HAVE NO CLUE of what I did that night that I was so amazing... But I would like to try and do it again sometime. I dont know when I'll meet her but I need every possible tip.

Another thing, usually when I cum, my penis is getting soft in a few seconds but at that night he kept on being hard for a good minute or so. And that happens only when I masturebate... Or when I get a BJ.

Thanks :)