I learnt about this as a teenager when my gran tried to kill herself (about age 75). She was miserable, a bit of pain here and there but nothing major, my grandad had already died, she had nothing to look forward to. Because it sounds like depression I'm not sure it would be granted even if euthanasia was legal now, but I know in my heart that the kind thing to do would have been to let her make that choice. She did herself some harm in the attempt, which she would have been spared if there was a painless and legal option. She spent another 20 odd years living a life she didn't want to live until eventually she died.
We can mostly agree that serious illness, such as terminal cancer, is a reasonable basis for making the decision to end your own life. I'm going to add my gran's reason, which is just being old and miserable and deciding you've had enough - it's perfectly legitimate to know that your life isn't going to get any better when you're that age. I would want the choice for myself at that age, and I would want that choice for my mother, who is getting old now and if Parliament doesn't get a move on she won't have that choice either. We put animals down and call it doing the right thing. I can't justify treating our human family members worse than we treat our pet family members.