I've known many, unfortunately. My best friend took her life when we were 15. She and I had a lot of in depth conversations about it, but she had always said she wouldn't do it. Her family was very kind but unfortunately very strict, and in her culture arranged marriage was what was expected. She did it in part to escape from that. I still miss her every day.
A mutual friend ended her life afterwards because of her passing. She also had always struggled, and while we weren't close, we did grow closer after our best friend. Her twin brother and I unfortunately lost contact, but I think of him often.
My husband's cousin who we were both very close with took his own life after a bout of psychosis. Schizophrenia runs in his family, and unfortunately by the time we all realized what was happening, it was too late. We had never spoke of it, but we talked about mental health fairly often... I know if we had gotten him help sooner, it wouldn't have happened.
And finally, the person I thought of more like a little sister than just a friend passed a few years ago after an extremely traumatic experience. Her life and mine were very similar, in terms of the things we'd been through and our physical health issues. I wasn't surprised when it happened, only wished she'd given me the chance to fly out and see her one last time. Or at least not have to go alone. I miss her more than words can describe.
As for how it has effected me... that's hard to say. I've never blamed them for what happened - blamed external things, like the failure of the healthcare system, or the people who hurt my friends beyond repair, but never them. They knew that, I think. What it has done is give me a better idea of everything I would like to have set before I CTB myself. I constantly wish I had more of my friends to remember them by, things to hold, pictures... that kind of thing. For example I used to be really averse to pictures of myself, but since my last friend passed I've made sure I've taken more with the people I love so they'll have those. It makes it a bit more daunting if a task, because I feel like there's so many steps before I'm satisfied, but I at least feel I'm doing the best I can to make it easier for my loved ones.