I don't think you should blame yourself for her death, she was in her final stages, and you would have felt worse had she been in severe pain while she was dying. You did what a good daughter should do for their mother, you loved her until the day she died.
You did the right thing. You eased your Mom's pain...remember that it was God who wanted her. He makes the final decision. I know how hard losing parents are. I pray for your strength, comfort, and peace.
I am so sorry for your Loss it feels dreadful doesnt it and there is always guilt,Sounds like you did wonderfully for her and all the visiting you did , this would have made her final years better. I dont think you can cope with the feeling you just endure it, let it flow over you and it will get better and easier, you couldnt have done more, I wasnt there for my Mother as you were for yours, and the Guilt never leaves me, I endure it , for me it wont get better but I have learnt to live with it, you did what I had wished I had done . Let us know how you are doing.