To lose a close friend to suicide is a terrible thing to have to bear, and often can lead one to feeling one might have done more, been more effective, said something different or a whole raft of other emotions. Grief, loss, anger, blaming oneself and feeling a huge void in your heart are natural.
I'd have to say I think you have done a mighty job. for over a year the sound of your voice, your thoughts and care have made a difference, have maybe giving him another year of life.
The fact of the matter is no person can keep another alive permanently by themselves if they wish to take their life. It needs a whole load of people, those close, a medical team and especially a spark inside from the person in question. All play their part.
You may feel he broke his word by not reaching to you that last time. I have been there and it is not the case, in fact one can feel one is releasing someone else by departing. That is a burden not all understand.
I'm glad you have met Sophie_M, her support suggestions are good ones.
May I ask if you have personal support? Maybe a family member or friend you can talk about this, your anger and sorrow, your efforts and loss? They do not have to do anything at all except listen and care - being alone is extra hard.
We are here too, talk whenever you like, it can only help