I've always held out for my mom to just kick the bucket. But despite a lifelong pack-and-a-fifth-a-day habit, she hangs on to this day. Also, it probably still wouldn't work… That little voice would always be there, doubting my every move.
Probably the only way that will actually work, is to face up to your parent(s), let them know about your fear, and try to work with them to modify both real and perceived expectations between you. In other words, to kill the fear of (or even the actual) disappointment, you need to communicate and manage expectations.