Welcome. Firstly, know that you are not alone. Your loss is still very new and grieving is a process... a journey to more love and healing. Each one of us goes through it at our own pace.
Was your mother in hospice in the end? You will find a tremendous amount of information and support here through the articles at the site under spotlights, death and dying articles, and even in some of the journal exercises. When one is alone as a caregiver, we can blow things out of proportion and stay focused only on the negatives of the process of dying. If you are able to, get a journal or notebook and begin writing everything you are feeling. Try to do it at the same time every day. It's an extremely cathartic process in that once you acknowledge your feelings, you can release them more and more.
Work on focusing on the good things that came from caring for your mom. The good moments. Caregiving is a gift that you have given to her.. and regardless of the circumstances, her soul knew you were there for her with love. Sit still, breathe deeply and feel her presence... she is there with you... and there will be signs that she is... but when you are traumatized, it is difficult to sense her presence.
I don't know where you live but it may also be a good choice to join a live grief support group offline where you can share with others and learn how they are experiencing and healing from their loss.
Your mother lived to a wonderful age and we all know we must go at some time. No matter what we think, it is always the right time... something we truly have no control over. There is a higher power or order in the Universe.
As for drowning... she may have been afraid to die. My dad was afraid to live and afraid to die when he became ill. We each have our own journey and we experience life as we need to.
These are all important things for you to think about. We are here for you. My your healing begin to be more gentle and nurturing for yourself. Begin to care for yourself as you did for your mother. Be gentle and loving to you. You have been through a difficult experience that is unique for you. Reflect on the good times as well and know that you are loved.