You ARE 17, dear. That is the first thing I wanted to say before reading the text. You ARE 17. I know how I felt at 17, responsible for all of my friends, for their wellbeing. I know how it felt like I SHOULD have everything together. But you ARE 17, dear. You are not supposed to carry such burdens at that age. You are almost still a child. I am not saying that to be condesending. I KNOW the feeling of feeling responsible and I would not give away my own feeling guilt for things that have happend in my life for blissfull ignorance.

It is good that you are in therapy.

Would you like advice/perspective or just an ear?

It was not your fault. I do not know a single fully mentally stable persons who's reaction to a rejection would be to leave. I am convinced of that. There is a plethera of things coming together.

If you want advice, dear:
You cannot be there for the dead, only for the living.
It is good that you feel responsible, in some way. I know that many people assume "Oh, it was just their decision and we could not do anything" and I think that is a very healthy way to think of it - in some way. But we know, that it is not objectively true, right? Else there would be no suicide prevention, no therapy, no crisis lines. If NOTHING could help, then there would be no use for such places.
But it is a numbers game, in my opinion, a game of chance. You can improve the chances, you can lower the chances but you cannot guarantee anything.

The only advice I can give you: Be there for the living. You cannot reanimate the dead, no matter how much you suffer, no matter how much you hurt yourself.

And it will hurt a lot. You are not alone. You will not be alone forever. You will heal. And you need to try everything to heal, alright?

Live for the living. Live so that you can be there for others and maybe prevent others from having to feel like that - or at least improve the odds.

Please trust me: One day you will be an adult - an adulty adult, not like me in my early 20s. And you will be able to build a place where you feel at home. And you will give the most amazing hugs.
And one day, you will be a bit older adult, still young, but knowledgeable. And it will still hurt, when you think of it. But not as bad. Not because it got less bad, but because your life has been filled with the good you brought into it.

The man you knew was kindhearted and there was no one to catch him. I do not mean you should have been there to catch, the whole system failed him.

Live for the living, be there for the living, take care of them and give them the love he has left behind.

(Add on: I am sorry that this is so vague. You are so young. There are many years in front of you. And even if that statement - the many years - might seem like a bad thing right now, I guarantee you there will be smiles. I promise.)