Standing sex

Maybe get a prescription to sildenafil and use that a few times to give you the confidence you need to do it on your own unassisted.

 

Maybe it's because you used to fap while lying down? Perhaps you should practice M'ing every now and then while standing (without ejaculation). No need to do anything crazy, perhaps do it only when you feel a bit horny and don't worry at all about keeping it up. Just give yourself a few tugs and then stop. Over time your body will get used to it I think. Just obviously do this in private, and take care not to let it become its own addiction.