Would your friend blame you? If the situation was reversed, would you blame your friend?
Your feelings are understandable, certainly. I think anyone would feel guilty. But you also know that it's not right to hold yourself responsible. You might try writing your friend a letter, or writing yourself a letter- but you definitely need to forgive yourself. You might consider seeing a grief counselor also, for help working through it.
I am sorry for your loss. That would be very hard.