Just keep living. If it truly because too unbearable, use constant drip sedatives to basically put yourself into a coma. If they, for some reason, stop giving you the sedatives, you wake up, look around to make sure nothing interesting happened, and then find a way to do it again.
At least, that's what I would do, though I doubt living forever would get that boring to me. Being a history teacher, I'm always interested in what's going to happen next.
Interesting question: What happens if you don't want to die, but world leaders nuke the planet clean anyway? Do you then die with everyone else or is the deal that you had to somehow be involved in their deaths to count?