Condoms just aren't as good as bareback. I'd try the thin poly ones. I had a positive experience with them for over a year.
I'm wondering if there isn't more to you going limp than just the condom. I'm guessing you haven't but I'll ask anyway: Did you have this issue with other partners? And even if it's a chainmail Trojan lined with treebark, I'm usually so aroused that I can recover for round two with a little manual or oral stimulation from my partner. Do you get nervous or frustrated? Nothing kills the obelisk of love quicker than being pissed off.
...
And you said loose but...
just how loose?