I'm going bowling without my girlfriend
Do you really want to go bowling without her?
I'm not so sure about it, but I'm afraid to tell her about it.
It's fine to do something without your partner every once in awhile. Doing everything together can be tiring.
I know but I'm a terrible liar.
Maybe you should just be honest with her then. Tell her you want some alone time. She's dating you because she loves you and I'm sure she won't mind that you want to do things by yourself every now and then.