From someone who recently went through a similar scendario, you did the absolute right thing by Casey, and Casey knew it, too. I know that you feel guilty, and tend to think of how Casey was at the end. The guilt will finally go away, especially if you compare the "normal" Casey with Casey at the end, then you realize how downhill she went and how right you were to help her have a peaceful end. Try to think of the "normal" Casey when you remember her, it will help some, and try not to dwell on the end. Talk to people who love you and who loved Casey, and don't be afraid to show your grief. It does get easier, but you always miss her. Try to keep busy, it will help. I'm sending lots of hugs and good vibes your way.