Another thing the guys might be using is trimix or quad-mix. I hesitate to even mention it since I do NOT recommend it, but it may be how the guys stay hard for hours on end. It is injected directly into the dick, and therefore is not for amateurs. I used it once on a porn set and it did nothing for me, but one of the other guys had a hard-on that lasted for hours. Long after the shoot had ended and the rest of us had gone home, he was still hard so they had to take him to the ER for an antidote.
In my experience, viewers are more interested in the cum shot than they are in having your dick stay hard. My shows are solo, so you may find the audience watching you with your GF are less interested in the cum shot, but I don't know. For most of my shows my dick is just semi-hard, and viewers don't seem to mind.
Between Viagra and Cialis, Viagra comes on stronger and faster but also wears off faster. Cialis is designed to keep you "ready" for a longer period of time. Cialis is sort of there in the background, and kicks in when stimulation is applied. But all that said, I don't use either in my shows and wouldn't recommend them. More important is to have the right lube nearby. I also keep some stimulating porn off to the side in case I need inspiration, though that might upset your GF... again my shows are solo so that's what I've found I need to do. You could also try a cock ring.
But remember performing on cam should be something you'll find fun and stimulating in its own right. If that's not the case, it's worth asking yourself why you want to give it a try. As
@Miss_Lollipop and
@Nigerian Prince mention, don't put too much pressure on yourself, and it's not worth your physical or mental health. Though this is work, it should also be fun.