Being the underdog is a strong source of energy. And you might be able to use it for a long time as your main source of motivation.
But if all goes according to plan, it’s ultimately unreliable. If the energy of the underdog is what’s giving you the juice to win then what do you do when you’re fully expected to win?
If you’re using a temporary source of motivation, keep going with it as long as it’s giving you what you need.
And then be prepared to find something more sustainable.