How to deal with my crazy-cat-lady Grandma. I'm getting resentful.?

I think you should just be straight (but kind, because the current living situation makes your life a lot easier) and tell her that with all the other stress of being a single mom, worrying about your child, finances, etc. having to cope with the stress of this cat making your life more difficult is more than you can deal with at the moment. I take my hat off to you and all other single moms. Take care and I hope she's understanding.