My deep condolences for demise of your mother and may her soul rest in peace. I can understand through what trauma you have been as mother makes you feel protected and loved. But her condition was grave enough to overcome it and you just helped her go easily through it by signing that. Don't blame yourself as keeping her in such situation for long would her given her more physical pain. Your loss was grievous and it would take you long to get out of it. Go to a good counsellor and talk out yourself, it's necessary for you and you should stop blaming yourself. Come out of the guilt as it was not your fault. Do meditation, yoga, exercises on regular basis, distract yourself and indulge into something of your interest. Mothers are always great and they can never blame their children for anything,