I think I'm going to cancel my husbands party.
That's a pretty drastic thing to do, what makes you want to cancel?
I don't want to spend time with his mother who will show up if I do throw the party.
Not liking his mother is fine but you shouldn't cancel his party just because she's going to be coming. You should just try to be respectful and tolerate her. He wouldn't have invited his mother if he didn't care about her and I'm sure you trying to be nice would mean the world to him.
I'm upset that I still have to be nice to her after everything she has done.
It's okay to be upset a little, as long as you're still willing to do it for your husband. It sounds like you're open to still throwing the party and I think that is the best choice for your marriage.