Hi L. ~
You are getting a lot of great advice here but I wanted to respond with experience behind mine.
I am a 32 y/o (I mean, "29") mother of 2 - 6mos. & 11. I have a mother that is very controlling as well and feels like she has the right to pick up her granddaughter whenever she feels like it, OR, show up at our doorstep because she "was in the area." Part of her sense of entitlement comes from the fact that I was a young mother to my oldest - 20 y/o - and had to move back in w/her for a few months until I managed to get out on my own again and she feels a special bond for my oldest since she thinks she saved us in some twisted way. It wasn't until I married my husband 2 yrs ago when he pointed out her guilt trips and controlling nature was unacceptable. I got guilt trips for anything from missing my great-uncle's birthday party to not going to attending random pool parties of theirs. The icing on the cake was when I came home from shopping one Saturday afternoon and found my mom and stepdad in my living room visiting with my daughter while my husband napped upstairs w/o any knowledge of their presence. I lost it and kicked them out because I was tired of her always barging into OUR home. (Of course, when she tells the story it is much more colorful and I'm an evil daughter.)
Right now, it is going on 3 months since I've spoken to my mother because of her manipulative and bullying behavior. I finally put my foot down and said no more - you are not to come over here w/o adult approval first (she used to use my daughter as the authority on that) and the guilt trips have got to stop as I AM AN ADULT NOW. Some mothers just don't want to let go and I think you and I both have one of them. Ultimately what I'm trying to say is, sometimes silence is the best way to get your point across after you've drawn your line in the sand. Mothers like those have to "feel the burn" before they get it. I'm happy to say, even though we're still not speaking and it IS tough on my oldest, it DOES seem to be working, slowly but surely. Stand your ground, explain it to your kids if they're old enough to understand and hold your head high because YOU will feel that much more in control of your life - I do.
I hope my experience is at least a little helpful for you. I KNOW it's not easy but we're not their little girls anymore and like we had to grow up, so do they. If you want to chat further, please contact me. Good luck!