Quitting smoking.
I've said it before and I'll say it again, quitting smoking isn't the hard part, it's the true committing to quitting that is. Most people struggle with that and then even if they are successful they're just one upsetting incident away from starting up again. If you can totally commit and say that's it, I will never have another cigarette, case closed, you can do it. If you really don't think you can, you're probably right.
I quit 25 years ago when my 2 year old son picked a butt off the ground and pretended to smoke. That was it for me, never smoked again.