Okay, someone needs to knock some sense into you.
FIRST OFF
You don't want to tell her the L-word?! Why the hell?! If you love her, then tell her, cause TRUST ME I've been through that shit. It's the WORST when you KNOW a guy loves you and he doesn't tell. That's when you get those bugging thoughts "why?" "what did I do wrong" "why won't he tell me?"
etc. It must be really crappy for your girlfriend to cope with her feelings especially, if she loves you. (and chances are, she won't tell you first!) This might be the reason to why it's not working out.
NEXT
Dude, you ain't kidding nobody saying that you'll be hanging out as friends, especially of you bring your girlfriend around too. This other couple will probably recall all the sweet and romantic times the four of you spent together and that'll leave SO much room for sentimental memories and a flirtatious mood. Your girlfriend will definitely HATE you after this.
AND FINALLY
Dude, double dates are for LOSERS!!! (Guideline # 1 for the perplexed) Seriously, that's so dumb. Only people who are excessively awkward/uncomfortable/unexperienced in each other's presence go on double dates! Just like back in highschool.
PS - Have you considered bringing a CHAPERONE with you on a date?
I have it all. Including kino.