Sometimes people achieve their goal only to realize that happiness didn't follow.
Of all the people I know who have made a similar statement and then found success, this was the most common outcome.
Being happy can be a surprising amount of work, and there aren't any instructions. It's better to figure out how you can be happy while also spending time on the goals you think are worthwhile. If you need them, there are other tricks you can use to motivate yourself that don't involve deliberately sabotaging your own well-being.