Nic, never feel guilty about putting distance between you and your mom; I do it quite a bit. I love my mom, but sometimes her negativity is too much for me to deal with. It's easy to come up with an excuse to "have to run" when the conversation turns negative. It takes some practice, but I have avoided many arguments using flight over fight.
I have come to the conclusion that arguing with my mom (and several other family members) is useless anyway, she is so set in her ways that my point of view is never even a serious consideration to her. Sometimes you just got to love them, you don't have to agree with them!