I don't think you should kill yourself, most people who try tend to regret it later, things get better. First try therapy, it has helped thousands of people become happy, you aren't the only one who feels like this. But hypothetically if I had an incurable disease and I wanted to kill myself I would use a suicide bag.
Heroin Overdose... Easiest and pain free method By Far...seriously.
Jump off a high place.
An easy and painful way to die is wait until you get old and die in your sleep.