Jb she was still your mom and I know this is a very sad situation for you. You didn't much say hello in later years, and didn't get to say goodbye. You have no feeling of closure. I would suggest getting some pictures together and making an album.
the only thing you can do is accept that she was who she was, and this cult no doubt made her worse, and it was not your fault. I hope she rests in peace and you can take whatever happy times you shared and remember them well.