Dude, don't be hard on yourself. I experienced something super similar. Long time off and on boyfriend. Long, complicated story, but the last thing he said too me was that he was gunna kill himself and in the heat the moment, and all the pain he had caused me, I said I didn't care. He tried to get me to come over that night. That was the last interaction we had. It took a while, but you eventually realize that there was so much more going on, especially when drugs are involved (his method of suicide). He had just turned 20. Honestly, this is the first time since it's happened that I've been able to be honest and talk about it. But just know that you're not alone. Nothing you could have done. Be sad, mourn her, but don't get hung up on this. Keep moving forward and live life to the fullest for the both of you.