In DBT, it's called radical acceptance. There are just some things in life that we have to accept for what they are.

I understand why you are feeling so bad about this. It is sad that she didn't qualify her statement when she made it initially.....but NOW you know that her implication when she says that she will bring you something is that it only means if there is anything left over. Many times people hold internal implications inside because it's just what they have lived with all their lived & think that because it's always been something that they have realized as their reality, it's not the same for others.

Passover is a huge family celebration....it doesn't surprise me that with her trying to get everything ready for it & focusing on the huge celebration that remembering to save you a matzoh ball was not high on her memory at the time.

I do think that next time she offers to bring you something like that, I would make the comment "you mean, only if there are leftovers!!!!" so that there isn't the misunderstanding the next time since it seems to NOT me something that she is used to qualifying her comments with. Clear communication is very important even for a T because it's the implied things that tend to be the things that hurt others.