Your mom had a lot of problems that should not have been yours to suffer for. It's great you remember the good times. That's all you need to do. You can't be sure that being with her before she died was going to be a great reunion. It may have been a disaster and her passing the way she did was right for both of you. Once something is done and over we mustn't have regrets that eat us up. Keep seeing the good.
I enjoyed reading your story even if it had some sadness. What you did for your mom at one time was very special and kind. It reflects the kind of person you are and your mom could only be proud of you for what you've become. Yes. You will need to grieve and you have every right and reason to. We only get one mother.