Mother-In-Laws

Soooo... my mother-in-law is an extremely negative person. She causes me daily anxiety by pushing her misery allllll over me constantly. I have a natural urge to want to help her, but I don't know how to not let it consume me and my family time. She manipulates everything and is so over bearing. It's making me resent her. We have even gotten into before like REALLY BAD because I can't stand her anymore. My husband and I and our precious little 11 month old couldn't be happier as our little threesome. I just wanted some advice as to keep her in her place and to make sure I can deal with this forever! I wish I didn't feel like I constantly needed to put my foot down, but I do.  It's starting to really affect my relationship with my husband. HELP!