This is going to sound horrible I know, but we think of desperate things when we're at the end of our rope, but I have actually thought about trying to give myself cancerous cells, and then refuse treatment because I want to try natural therapies or something. It would be so painful though, and instead of my family having to deal with a sudden suicide, they'd have a drawn out and slow death, which is probably more agonising for both of us.
How would I go about doing it? Find radioactive stuff and carry it around with me, sleep with it near me, whatever it took, build up a little stockpile by taking stuff apart that has that material in it, and let it bombard my body with radiation until my cells mutate and get cancerous. Hell my family even has a history of it so the doctors wouldn't even bat an eyelid if it worked.
Morbid, painful and probably unrealistic, but it was something I put some time into considering.