Strange erection problem

Hi everyone, and thanks for taking the time to read my post. I'm 26, male, and I've rarely had a succesful intercourse in my life. I can get an erection quite easily, and when I am alone there is no problem in maintaining it whatsoever. When a partner is performing oral sex on me, there are no problems either. The problems occur either when I try to put a condom on, or while penetrating my partner. I just can't seem to maintiain an erection long enough, despite the fact that I find the experience very enjoyable.

I am generally a stressful person, and I believe that this plays a big part in it. With my previous partner we were in a serious relationship that lasted a few years and after a while, only with her, I did not have any difficulties. Now though, we broke up, and I'm terrified at the idea of facing a new partner with this problem all over again. I know that people are telling me to relax and don't worry about my perfomance, but I can't do that. Even the thought of seeing again that look of disappointment on the face of a potential new partner breaks my heart, and I don't want to go after my previous partner only for this reason. I have no doubts about my sexuality, and I never had a traumatic experience. Trying to meet a girl that is willing to settle with mutual oral sex might in theory look like a nice idea, but like I said, I don't want this to be a reason for me to be with someone. I'm out of ideas and almost desperate, and i'm really looking forward to some advice.