Ha! I love milenafk's suggestion above!
I have to say, though, that I think forgetting the actual number isn't important. I haven't spoken to my ex since...August? His number is still in my phone, with his actual name. (I leave it there so that I won't accidentally answer a phone call from him.) But the desire to call him, text him, talk to him, is dead.
Just give yourself some time. It takes a while.