Set one tiny private deadline and hit it. Self-trust gets built in sizes nobody else would notice.

Trust in yourself works like trust in anybody: it comes from a history of things said and then done. Large promises are poor material, since they are easy to make and easy to break, and each break teaches you that your word is provisional. Small ones are better precisely because they are too small to fail.

Pick something nearly trivial and put a time on it: dishes by eight, run before dark, one page before bed. Then hit it exactly, with no renegotiating halfway. Do that four or five times and you start believing yourself about larger things.