mwwa, I agree with the others in that it seems pretty clear you're working your anxiety up to boner-killing levels, and are hoping that a drug will cure you. Like skyray said right up at the top of the thread: It won't do what you want it to.
Trust someone who has experienced a lot of anxiety in his life: It'll kill your erection, especially when you get into a self-perpetuating cycle like it sounds that you have. You get anxious, so you lose your erection, so you fear that you'll lose your erection, which makes you anxious, so you lose your erection. Wash, rinse, repeat.
The thing is, mwwa, it sounds like your anxiety is learned, which means you can un-learn it. You do it by riding out the anxiety. Anxiety, like hunger pains, cravings, or happiness, comes in waves. It's both a mental and a physical thing, too. You get all worked up and you body gets flooded with stress hormones. So it will take some time for you to ride out the anxiety.
The best advice I have for how to do that is ignore your dick for a bit. Tell your girlfriend that you want to just focus on her until your dick comes around. Eat her out. Kiss her all over. Let her kiss you all over. Find out what makes her really, really hot. Eat her out some more. Tell her how gorgeous she is, and how hot you are for her. Tell her how great she tastes. Take some time doing this - ten minutes or more. (I doubt she'll say no to fifteen minutes of you worshipping her body.) Give some time for your brain to spool down and your body to flush those boner-killing hormones.
If you can ride the anxiety out, you'll be able to get your boner back. It might take some time, and it might take repeated tries, but it's the best advice I have for you. And after enough tries, you'll teach your body that there's nothing to be anxious about.