You understand what the pills do, right? They open up your arteries to allow more blood flow. An erection is simply the penis engorged with blood. The pills don't make her more attractive to you. They just make it so your blood flows more freely.

Why would your blood not flow freely? Well, there are physical and mental reasons. Most of the physical reasons are seen in men who are a bit older (but not exclusively). They involve some clogging of the arteries, prostate issues, high blood pressure, etc. One mental reason that is common is stress. Not just stress from work or family, but any stress. The more you think about things the more you stress.

You know why alcohol works so well in getting people to hook up? Because it relaxes them. It lets them act without stressing, without thinking.

Now, I want to quote something you said:

i just feel like she would be less attracted to me if she knew i was getting chemical help for erections...is that an irrational fear? she is very supportive of my therapy and has even offered to come with if i wanted her to...
A couple points out of this - I can't say it is an irrational fear, because you know your gf, not us. But it is a fear you have. And it is a fear you have created yourself from thinking and overthinking and overoverthinking. That type of stress is doing you no favors. Sexual activity is instinctual. You have to find a way to tell your brain to relax. The second point is that you have said nothing but good things about your gf. That she is understanding. That she is supportive. That she wants you. That she wants you to want her. Every thing you have said you worry about her feeling seems to have developed from your own head, not from experience with her. So, because of that, I will give you this advice:

Trust her.

Talk to her. Tell her whatever you feel like you need to. Take her up on her offer to go to therapy with you. Or let her know you will tell her all about it if she wants. Tell her you are sorry that things are not perfect, but that you love her and do want her very much. Tell her you have some Cialis if she would like you to try it. (You can leave out the fact that you have before and just say that a doctor suggested it might help previously but you've been scared.) But in general, trust her. And in this situation, trust her over the enormous amount of pressure you have built up in your own mind about this issue.

Good luck.