My sincerest condolences. Please, don't feel guilty. As your mother hadn't recognised you for five years due to Alzheimer's there is no way she would have known you had you been with her when she died.
My mother died 15 minutes before I got to the hospital and as she had been in a coma for weeks, I consoled myself with the fact that she would not have know whether we were there or not.
My father was in command of all his faculties, but died five minutes before we arrived at the nursing home, he had understood we were coming, so I felt he had chosen to slip away before we got there to save us a distressing scene. Nonsense? Perhaps, but quite probable knowing my father who detested "fuss and bother".
Take time to grieve, we all have only one mother to loose, even although some, like you, loose her twice, due to her not having been able to recognise you for so long.
Now she is free of all her trials, so I hope you can feel glad of that. Allow yourself to be glad that you no longer need worry about her. I am sure the mother you knew before her illness would not want you to feel guilty, and you certainly have no need to.