Please, forgive me right now, because I'm sure this will rub you the wrong way. In fact, reading the comments, this will probably rub a lot of people the wrong way. I'm sorry, I truly am.
However, in my humble opinion, you should not take it personally, because this is not about you.
Your husband is grieving. It doesn't matter that his mother is a horrible person. I could try to justify him by saying that it's his mother, so he'll be sad no matter what, but I won't, because it doesn't matter WHY he's sad, what matters is that he IS sad. He doesn't have to justify his feelings. He's in pain. He's hurting. Period.
And yes, you are right: his mother is the worst, and you have been always there for him. But you know what? Grief and pain do not care about that. Grief and pain are there, and trying to battle with them by saying that he shouldn't feel those emotions because you are there does nothing to help.
"But wait," you say, "that is not what I meant!" Well, does he know that? Why you're sure he meant to say that you don't matter? I can't say whether you are right or wrong to think he meant to put his mother above you, because I cannot ask him what he thinks, and so will never know. However, YOU can ask him- preferably in front of a therapist.
Think of it as if you were approaching someone with depression (which, from what you've been saying here, might not be too far from the truth). They're depressed, and they feel guilty about being depressed, like they feel guilty about making other people worry/angry/sad for the fact that they're depressed, and the fact that they are feeling [insert emotion here] for something inconsequential in what might be an otherwise pretty good life/not as bad as other lives makes them feel even more guilty for being depressed. None of it makes any sense, and that's just how it is.
You can't go to a suicidal person and say "but what about me?" and think that solves every single problem they have.
So, in conclusion, my thoughts are these: he's hurt. You're hurt. Talk about it with a therapist.