Those kind of thoughts aren’t really fear of failure, and you can’t just make them go away. If they burden your life, you should probably see a psychiatrist about involuntary negative thoughts.
If they don’t burden your life, think them through. Is it true? Would nobody use your product? Would you? Ask someone else if they would.
Another good thing to start with is the why. Why are you making the product? Are you trying to become rich? Are you filling a need? Are you doing it because it’s fun? Are you learning something?
Everyone fails though, what matters is that you admit it when you do.
Knowing when you’ve failed can be a bit tricky though, especially when you haven’t made an exact mistake. So set some goals for your product, and evaluate how you meet those goals, and if you don’t, evaluate if it’s worth setting new goals or trying something else.