Kiwi, sorry to hear you are so down. I, too, am trying to keep my head above water with ttc.
I have the same issues as you at the moment. Eight of my friends have announced pregnancies in the last few months. EIGHT!!! I'm really happy for them, but I find myself saying "oh you are so lucky, we haven't had any luck yet". It's really difficult to not make it about yourself. I agree with the above - you might just have to tell your friend you are ttc yourselves and not having any luck, and though you are over the moon for your friend, you find it all too difficult to talk about at the moment.
Good luck and be honest with your girlfriend, I know she'll understand.