That is hard...my mother was ill for some time but had steadily improved and then, very suddenly, an “event” and death within a day.  So sorry for what you’re going through.  People often seem to feel that once we are in our fifties we should be prepared for our mothers’ deaths, even that we should be grateful that it was sudden, and in a way I understand all that, but it isn’t true, at least not always.  We were not prepared and the loss, though I suppose it is natural, a part of life, is terrible.  I hope you have beautiful memories of time with your mother, that is something that comforts me.  Wanted to mention that there are groups on this site focused on this kind of loss, so you may want to look at them, sometimes reading about other people’s experiences of similar loss can be helpful.