Ok, I'll say what I think, but I have no idea, will it help you or will it make things even worse.
So read on your own discretion.
When my depressed friend commited suicide (and I, obviously, didn't), I felt like it's unfair, too. I felt guilty that I made through this, and he didn't. I felt that it's horrible that I dare to live, when he doesn't live anymore.
What I feel now, after four years (or so)? I still feel it's horrible. Yes, it's very sad and unfair that people we loved die, and we continue to live without them. But it's how this world functions. If you don't want to go away with them, but want to continue living, you have no choice besides let them go and find new reasons to live, even if you feel that it's unfair. It's ok. World IS unfair as a matter of principle. All I can suggest, is accept it and live with it, even if it's unfair.
If you feel what I felt, maybe you imagine in your head, how different everything would be, if you would behave even more better with the person you lose. How would it be, if you could give them even more warmth and support. How it would be, if you suggested help even more persistently. How it would be, if you would pick even more right words. How it would be, if you could look at the bottom of their soul and make the most possible effective appeal to their heart, which would make them change their decision.
Probably, you (and me) really could behave better and could make more for them. Or probably we couldn't. You will never know exactly. And even if you DIDN'T do ALL what you could, it's ok. Nobody is perfect, nobody can give perfect empathy and pick ideal words. You are not God, you are just human. Don't take a responsibility of God on yourself! You've tried to help, but you couldn't do it ideally and couldn't guarantee success because you are just a human, you are not ideal and you can't know how to behave ideally in one situation or another. So I'd suggest to just absolve yourselves of this responsibility.
This is probably the most controversial point, which I will make... but still.
She have done what she thought will be better for her. She made her own choice about how and when to end her life. And she had perfect right for it. I think that everyone should have a right to take their own life and death into their own hands.
Of course, you can say, that her conscious and mindset were skewed by the depression, and she could be very wrong, and make a big mistake. And this is true. Probably it was a mistake. But what if it wasn't? What if it really was the best for her? Of course, her had cognitive bias, caused by depression, which skewed her way of thinking. But who is free from cognitive bias? Nobody. People always make their choices with emotions, which aren't always right, in the condition of incomplete information, and in the condition of skewed worldview - in one way or another. But, nevertheless, they do what they think is the best option, in one way or another.
She had her own will, and it was HER and only HER CHOICE, right or wrong.
You can't make choice for other people. You can just try to help them to make a right choice (or what YOU think is a right choice!), but their final decision is their own decision. If they don't want to be saved... it is their choice. It' their right to not being saved, if they don't want it.
In this case, just let them go, no matter how sad is it.
P.S. I remember your post about the unhappy pregnant rat from the pet shop. How you said that "no creature deserves to live in such a bad conditions, in which that rat lived".
Long time ago, I tried to save every creature I've seen, too. But now I think: it doesn't worth it. Moreover, it's impossible.
You've saved one pregnant rat, but millions other pregnant rats are still suffering and dying. And not you, not me, not anybody can save all rats, or all people. There will always be cruelty and unhappiness in the world. I don't try to say that it was bad thing to save that rat or something; if it made you feel better, then it was good for you. What seems very problematic to me, is your justifications of this action.
You can't give everyone what they deserve! You are not messiah, you will never be able to save everyone. You can try to help, you can suggest help to distinct people of creatures, but you will never save everyone, you will never make the world ideal. You do not have the power to do it, and it's not your responsibility.
So, my way to deal with it is - throw away the White Knight's armor, throw away the messiah clothes and accept that:
- World is not fair and will never be fair;
- You will never be able to act ideally;
- You will never know for sure, what is better for other people;
- Choices of other people are their own choices, and you can't do them for them;
- You will never be able to save everyone.
I'm sorry if something from it made you feel worse, or if it was unpleasant to hear preaching of the person, who is younger than you. But if at least something from it will turned out to be useful to you, probably I didn't waste my time on this post in vain.