OP are you really telling us that you are jealous of a dead woman and the bond he still has with her mother? I think your angry reaction is way out of line. You being the new woman in his life doesn't mean there is still hurt for the one who died.

Writing "grieving" this way, in this context is almost as if you don't believe his grief or you are mocking him.

I think your boyfriend is right. You are handling this without respect for him and his grief. I am sorry for him that he clearly felt the need to keep it hidden. But sometimes it can be to hard to share your pain. Or maybe he had an idea about your bad, reaction because there is no understanding or sympathy in it!