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I am ready for my check ride, but I am still anxious before each flight. I imagine I will always be anxious before each flight, primarily because I'm not wealthy and may never have the ability to fly so frequently that it becomes second nature like driving. But my anxiety is not borne of fear. It's more an emotion that arises because I am about to perform a challenging act that has an element of risk. Once I arrive at the airport and begin pre-flight, my anxiety dissipates as I focus on my tasks. It arises again briefly a bit before takeoff, and then is mostly gone again during the flight.
So I'd like to know a bit more about what you're feeling when you "chicken out." Depending on how you deal with emotion, it might make sense for you to spend some time trying to understand what it is you're feeling, why you're feeling it, and then making a concerted effort to rationalize that emotion. If it's fear you're feeling, what you'll probably find is that there's some element of irrationality to that fear, especially in light of the fact that your instructor(s?), who likely have many years of experience, have signed you off to solo and must feel confident in your abilities.
Perhaps you can redirect the fear into a different emotion. I consider my pre-flight anxiety to be healthy because it's not debilitating and keeps me cognizant of the fact that I'm about to do something that requires focus.
Then again, maybe you're the type of person that needs to avoid overthinking it. If your instructor says you're ready, maybe it's time to take a leap of faith.