I deal with self loathing and generalized negativity about life as often as the next person, but it never actually takes the form of "I bet I wouldn't be good at ____". It's more like "will ____ turn out to be as unsatisfying as nearly everything else in life and do I even want to waste my time".
That's the interesting thing about having been considered gifted as a child. You get used to hearing how exceptional you are at everything, so even as an adult, it never occurs to you that you wouldn't be good at whatever you try. You just assume that you not only will be, but that you'll be better at it than everyone else. And you're even right a lot of the time.
When that's your reality, the act of accomplishing things is easy and belief in your own ability is not the problem. It's the act of mustering any kind of enthusiasm for 98% of the so-called opportunities life presents you with that is difficult. It's the remaining 2% that keeps me going, but holy shit... that 98%. The struggle. It is real.