Boundaries Shmoundaries

While I am rediscovering boundaries all the time, I realize that the ones I have in regards to my MIL lessen with each ever so brief contact.
I try to hold my disdain in check for my husband's sake but sometimes I find it impossible.
Example.......
We went out for dinner on Sunday to celebrate my FIL's birthday. We were to meet at the restaurant at 5. We naturally assumed they would be late as it is physically impossible for my MIL to be on time for anything. We were surprised to see FIL's car pull in right as we were heading inside.
Or we were until we realized that my MIL was not with him.
She had gone to mass instead.
For the 3rd time that day.
Yes folks, she followed typical fashion of bailing out on family to put in an appearance at church (where, given my admittedly limited experience with her at church, I am fairly certain her time was well spent sleeping/ talking on her cell phone/ writing letters home). My MIL attends every mass given and confession every day. If there is no mass or confession, she has been known to simply sit there and wait for it.
This is far more important than her husband's birthday, a visit with her grandchildren, making sure that her husband got to his very important cardiologist appointment (in her defense, she did have to give a ride to a complete stranger to church instead) and so forth.
It was no surprise at all though that she called just minutes before our dinners were served and guilted someone into picking her up (because driving herself would have meant that none of us cared about her. After all, this dinner was all about her).
I know that I shouldn't be like this towards her. But it's hard not to bitter towards someone who has said and done some very cruel things to me and my family.
I've said before, I've stopped caring so much when she calls me fat (Gee, being 5 months pregnant might have an effect on my body but I could be wrong), mentioned that she didn't want her son to marry me, called me sloppy seconds, discounted my daughter and insulted my parents. But I do. Partly because I always imagined my kids having grandparents they could do things with and we're stuck. My parents would be those grandparents if they didn't live on the other side of the country and I resent that instead, they get ignored. MIL only comes over for birthday parties and holidays. They live just 20 minutes away. We don't go over there because we value the health and safety of our kids and... well.... I don't know how to explain to my MIL that penicillin has already been discovered so it would be OK to clean up now.
But it also bothers me because of how it effects my husband and my relationship with him.
Because as I heard my SIL agreeing to come and pick up my MIL I couldn't help but roll my eyes and my husband gave me that pleading look. That one that says "Please, she's my mother. Please, just accept her".
Yes. Just accept her.
No matter how much she hurts you.
I'm trying.