i've had this problem for like ever. I feel like a lot of it was my catholic upbringing and feelings of guilt about having sex. You have to realize that it is not a physical thing. It's completely mental. I tried numerous things: supplements like gingko biloba, maca, tribulus, caffiene, ginger, stopping masturbating, stopping porn, working out, changing my diet, and on and on. These things will all increase your libido, but not NECESSARILY cure your PA. however, i do find that when i'm very horny it's not as much of a problemt because i have more confidence. Especially the working out part. If you dont already weight train (cardio, yoga is great too), you need to. I would recommend start working out, eat a lot of garlic and good fats, and maybe take a libido supplement. Even if it's just placebo, placebo can be very effective.
I'm in a LTR right now, and it's not as much of a problem, but it's something that is always there and irritates me. The biggest thing to do is relax and not be outcome dependent. Don't put any pressure on yourself to perform, and if your member does go down, tell the girl what's up and act like it's not a big deal. Tell her it happens sometimes, BECAUSE IT DOES. For me it usually happens during foreplay before it even enters her (condoms make things worse too). If i'm decently horny, theres never a problem once it's in and we're already having sex. You can always blame it on alcohol or stress or whatever if it's a new girl if that helps you...but if it's a recurring chick and it happens more than once, you should just be honest with her. While it's not exactly an attractive thing, many girls will be understanding of it.
I actually went to my doctor and got a box of cialis because it was bugging me so bad. Honestly, just having the pill there IN CASE is usually enough to make you not worry about it. (NO i'm not a viagra spammer, this is honest to God my experience). And a worry like this can impair your ability to flirt and escalate if you're too worried about how you might perform if you get this girl home with you....
I find that it's something that carries momentum: if you have a bunch of bad experiences in a row, then it starts to get you down. But if you bang like a stud a couple times in a row, it's much less likely to happen.
PM me if you have any questions...i'm in a similar boat