Is it possible to let you know that I forgot my wallet?
Oh my goodness.  What caused you to forget your wallet?
I was in a hurry and spending time with my girlfriend.
Not having your wallet when going out could be very difficult. While there are ways to prevent it, this is just something that tends to happen from time to time. Using some sort of finder alarm would be an efficient idea though if you find yourself misplacing your wallet often.
I wish I could have that. I'm so frustrated I can't believe I forgot my wallet.
I'm sure your girlfriend will understand, as you didn't mean for this to happen.  But there are some pretty simple tricks that you can use to ensure that it never happens again.  Make sure you have it on you as soon as you get dressed in the morning, that might help too.