Whatever you repeat is what you get better at, and a long run of abandoned beginnings trains you thoroughly in the easiest part of the job. The hard skills all live near the end: deciding something is done, accepting the gap between the plan and the result, grinding through the last unglamorous stretch. None of those come up if you keep restarting.

Count your finishes rather than your starts for a while. Before beginning anything new, take one existing thing to an end, even an imperfect one.