Sometimes I tell my girlfriend I have a bad memory so I have an excuse when I really forget something.
What did you forget about?
I forgot what I had to do that day
You don't need to make excuses for forgetting simple things.  People naturally forget stuff from time to time.  There's nothing worth hiding!
It's a bit shaddy and I dislike myself for doing that. I think I should tell her and apologize.
After you apologize it might be helpful to invest in a notepad or an app. You can use it to keep track of things you might forget, I find them to be quite helpful!