i know exactly how you feel. i know your feelings of guilt and remorse, that you should have seen something and you should have known. if it could be that simple, but it's not.
you are beating yourself up and that is going to hurt you in the longrun. it will slowly eat away at you until you are empty inside. your friend would not want that for you. whatever decisions led him to his decision are beyond our understanding. that is the unanswered question that will remain, but hopefully as time passes, it will fade a bit.
try remembering the good times and the fun. replace the focus on his death with a focus on the good stuff. i know it is hard, but once you get to that point, it is much easier to bear.
my son committed suicide. i know your pain and your journey. i beat myself up for so long, with "why didn't he tell me.." and "why didn't he call me?" and "what did i do as a mother to cause this" and so on and so on. there are no answers, just acceptance. i started down the path that you are presently on, and sweetie, it just doesn't work.
be kind to yourself. it wasn't your fault. it was NOT your fault. there was nothing that you could do - truly.
let me know how you are doing. my heart aches for you.