Hi Sizzling,
I am curious as to why you wouldn't tell her? Erection problems are quite common. She is your girlfriend and should be understanding to what you are going through. I know you are afraid of her reaction and she may he react irrationally at first but this problem affects you both so she has a right to know. And her knowing may even make you relax a little rather put yourself under all this stress of needing to perform. My kinda-ex (we are working on things) suffers erection problems and was put on Cialis and is also seeking therapy for the underlying issues as his issues are pyschological. If he had hidden all this from me and I found out later I don't think I'd be as open to reconciliation as I am now. He was truthful with me from the start and I really admire and respect him for that.
Good luck
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!