Finish enough things that your imagination learns you can be trusted.

Every abandoned project teaches you something about yourself, and what it teaches is that starting does not lead anywhere. After enough of those, your own enthusiasm stops being persuasive, and new ideas arrive already discounted. Finishing repairs that, slowly.

Complete something small, deliberately, even if it is not important. The point is the record. Trust in your own follow-through is built the same way as any other trust, through demonstrated behaviour.