I'm going to pretend to be asleep
Why would you pretend to sleep?
I don't want my girlfriend to know I'm awake
You shouldn't pretend to be asleep to fool your girlfriend. If there is a reason that you're avoiding her then you should tell her.
I didn't want her to think that I hate her, I just want to be alone.
It is wrong to trick people into believing a lie. IF you want to spend time alone you should tell her so she does not be come worried about you.