Welcome to SR! Zoey said it perfect!
And to answer your questions-
Why am I feeling like this?
It is natural feeling to feel like you are doing something behind their back and feel guilty but, IMHO you are doing something for yourself and your sanity and that is nothing to feel guilty about at all.
Is this normal?
YES! Totally normal to feel this way when you are involved with an A
Read some stickies-and possibly think about an Al-Anon meeting!
Good Luck and Welcome!