It is not for man to rest in absolute contentment. He is born to hopes and aspirations, as the sparks fly upward, unless he has brutified his nature, and quenched the spirit of immortality, which is his portion.

Restlessness is not a fault in your design. Getting what you wanted and finding yourself already looking past it is what minds do, and treating that as a failure of gratitude just adds a second problem to the first. Total contentment was never really on offer, and its absence is not evidence that something has gone wrong with you.

Since the wanting is going to continue, the useful question is what it gets pointed at. Aim it at something that improves in the doing: a craft, a body of knowledge, people you are building something with. Appetite spent on those wears well; spent on comparison it wears you out.