This post is a tad old... but here goes anyway. A few years ago, I had a similar problem and it all but brought me to tears. After starting over from a relationship that lasted almost a decade (with my partner using only IUDs), I used condoms to protect myself and found that I'd perform terribly.
I'd go from rock solid to wiggly soft within moments of putting-on a condom... and efforts to revive my erection only lasted briefly. I thought my problem was related to stress or anxiety... but it simply turned out to be the size of condoms that I used.
A spoke to a friend and pharmacist discretely one night and bought a bigger size based on her suggestion. I always thought condoms were made with a "one size fits all" approach and I was wrong. With the bigger size, my performance was much better. I was able to mainitain an erection and perform satisfactorily (in my opinion). So maybe all you need is a bigger size.
If you go soft before youput it on, then the problem may be psychological. If this helps, please post a reply so that others will be helped as well.
Good luck!