You're 30, guy. I'm 68, had prostate surgery, and since then it's very much hit and miss and as time goes on mostly miss. I'm sure a lot of that is psychological even for me. The meds are not risk free, as they do affect blood vessels -- that was their original purpose -- and can give you erections that don't go away. Get too many of them and you're in trouble. When you're young and strong and full of testosterone, that's probably more likely than it would be for me, but I haven't done it yet because if it works sometimes and if it happens often when I'm trying to sleep, that makes me worry, and again, I'm getting old. Look, you're posting over and over and over and that should tell you what's going on, as you don't appear to have any physiological issues. The fact is, men define themselves in way too large a part by their erections. It's still true at my age and it's much more true at yours. Men really have only two major contributions to life, which is physical strength and erections. Other than that we're more of a nuisance. Women raise the kids, have the kids, do most of the real work around the house, etc. etc. while most men sit around and belch. Now, this is simplistic, obviously, but not so much psychologically. Pulling our weight and erections are a core part of how we judge ourselves. That's what you're doing. Stop doing that and you'll be fine. Keep obsessing over it and you'll keep obsessing over it, get it? If you want to stop this and can't do it yourself, it's time for therapy. No matter how many times you post, this will always be your answer.
For you, I'd be afraid you'd get too reliant on the meds, psychologically, and then you'd worry that you couldn't get an erection without them.
Also, where are you getting them? These meds have potentially serious side effects, and if you haven't been evaluated by a doctor, you shouldn't take them.
You haven't yet ruled out physical causes, or gotten help for your anxiety. You should do both before taking these meds.
Have you talked to your girlfriend about all this? That may help, too.