That's I why I tend toward the Stoicism view of the world.

The tl:dr version is, discipline is a virtue. Your goal should always be living according to your virtues, therefore you achieve happiness through the act of practicing discipline itself, rather than the outcome of being disciplined. The idea being that you only have control over yourself (being disciplined) but do not have control over externals (whether or not the discipline produces the desired outcome). You shouldn't weigh your happiness on externals which you can't control.

This is a pretty poor write-up to be honest, and really only scratches the surface, but it might be sufficient to get the point across.