This is a good a question and it has a simple answer. It’s easy to believe in your work if you believe in your process. When I say process I mean some version of a traditional design process. A variation of identifying the problem, investigations, explorations, feedback*, refinements etc., we describe process in depth here. Whether you develop a process independently or through formal education, it is the foundation for trusting your work. Good decisions are cumulative. If you approach your process with rigor and sincerity, get and respond to feedback, and follow through to the end, the result will always be functional, it will solve the problem. It may not be exceptional but it will be useful. Creating consistently exceptional work is a separate challenge and a bit more capricious, but the first step is developing and trusting your process. Of note: every other word in this paragraph is “process.”
*Never scrimp or skip the feedback, it is vital to good work and improvement as a designer.