That said, the other issues brought up in other replies, still apply, especially if you are young and/or inexperienced.

Well, it's easier said than done. But from my experience, that's how I am able to do it. I also have similar issues of going soft.

Hi, i have a lot of questions today:D my cock is okay, it is hard normally, i can jerk off without problems, i like guys, why it isnt hard when i have real sex? like it is hard while chatting with someone and when we do the deed (only blowjobs for now) my cock is only half hard and afther while soft… do i have some problem or it is nervosity?

When you are jerking off, the only thing on your mind is your own pleasure, so one is more relaxed.

When you are with someone, most of your focus is likely on the person and what is going on, and the surroundings, and that is probably distracting. Its even worse if the person is a stranger you've just met and jumped into bed with. Ive noticed a similar reaction when I used to meet guys off the internet. It was exciting when we were chatting, but by the time I had showered and was going over there, or waiting for him, I had lost my excitement and was filled with more anxiety than anticipation.

Funnily enough, on the very other extreme, I also had a similar experience with the guy I was most in love with in life. I couldn't get hard with him and I realized that it was because I was totally into making him feel good. I was so focused on him, his reactions, and his pleasure, that I had little inner energy to spare for myself. The relationship did not last very long!

Don't put pressure on yourself, and enjoy the moment.