Dilbert is on the money mate, just relax about it.
Firstly, if you're not a heavy smoker, try giving it up, it doesn't do you any good anyways. And no, I'm not a preacher, I'm a smoker myself, but I feel soooooooo much better when I'm not smoking. If you are a heavy smoker, maybe think about patches.
Secondly, DO NOT TAKE VIAGRA AT YOUR AGE! apart from being completely unnecessary, it is also a health risk to younger people who have a perfectly healthy sex drive.
I think you guys just need to figure out together what will keep you hard, and have plenty of fun exploring
The chief piece of advise, as Dilbert pointed out, just relax about it dude, I'm sure you're girlfriend will have just as much anxiety about it as you do right now.