I hear what you're saying. Tough situation to be in.
But here's the deal: quitting isn't as easy as that and a large majority of the people who do quit often go off then and on them over and over again...some eventually quit permanently, some don't (and you should hope that you're eventually in the former rather than the latter).
Smoking to me has always been something I rely on in stressful situations, in situations where I just need an excuse for a break, as an integral part of my social life (drink in one hand, smoke in the other). It goes on and on. It becomes such a hard habit to kick simply because you have to change yourself so completely.
That said, I reckon you should explain to your girlfriend *exactly* what you've said here--might be a hard thing to do (and she might get ticked off at you for saying it) but I think you should at least try. Because maybe then she'll see the situation she's unintentionally putting you in, and maybe she'll also realise that this pressure you're feeling from her (even if she's not doing it consciously) is actually stressing you out and making you *want* to smoke.
I know plenty of smokers who have taken years and years to give up. And as the Australian "quit packs" say here: even if you don't succeed the first time, every day you don't smoke is a good one...and hopefully you'll get back on the wagon.
To make you feel better, ok, I officially announce: I AM SMOKING AGAIN! <sigh> Tell you what, Stonemaybe, saying I'm p*ssed off with myself about getting back on them is an understatement. And I'm going to try again...and soon... But
1) I'm telling you to demonstrate that it is hard...but I'm also saying it because
2) If you can stay off them, stay off them mate. I felt brilliant when I was off them! ...and I'm ticked that I did all that hard work only to give up because my life got a little bit stressful and I said "I'll have one"...and then several weeks later I'm back smoking like I used to.
If you can hang in there then do. If you can't, don't beat yourself up about it, just remember you've done it once (so you can do it again!). Hopefully your gf will understand! Sending positive vibes your way!!!