Bad on you for being in a relationship for any sort of length of time without making a point of learning - and remembering - her birthdate.
Hey! I completely forgot about my own birthday last month until a couple days afterward when I logged into one of my Facebook accounts and saw a few well wishes. Was just too caught up in my work (prepping stuff for sale, prepping sold items for shipping, going on a Jawa raid, aka thrift store run, for more stuff to sell), and didn't take note of the date.
I reckon that if I can't be arsed to remember my own day (not important, really), then it is best that I have completely given up on trying to be anything but single. Because who knows what more important things I would forget about, eh? Then again, I always figured that any gal who would be interested in me is either desperate or damaged anyway.