Self doubt and worry is self perpetuating, and the only way to deal with it is to stop the cycle. A few ways to do this include putting your worries on paper so you can examine them objectively, finding stimuli that block out the doubt by redirecting your attention, and working through self affirmation techniques even when they don't seem to have any immediate effect. It takes some time for a new way of thinking to really take hold within the brain. The old ways of thinking ("I'm going to run out of success at some point" or "I'll slip up and ruin things next time even if things went well this time") are extremely entrenched. It's sort of an "you are what you think" thing... and it's also a perception thing. The brain has to be retrained, in other words.