We stay motivated until motivation bears novel fruit. No better example to illustrate it than what new year resolutions entail. We party (or laze around) till December 31st night and then wake up afresh on January 1st to venture off to fulfill the new year resolutions. In the beginning you can keep your word because …
We can't be certain what the future will be, that's more of a reason to do what you want. Because once the time passes and the opportunity is gone, you'd wish that you had the motivation to achieve what you wanted the most, rather than what you have.
Hardwork leads to failure which leads you to success. Hardwork is the test ground of your effort and failure is the test ground of your motivation. Unless you pass both the steps, you cannot reach success.
It tells you how well one can imagine losing, not what the person can do.
It's what keeps you going.
Yes, there is some discussion about intrinsic and extrinsic motivation, how you are driven by internal and external factors to reach a goal. But how effectively the internal and external factors work on you depends ON YOU. If you're receptive to new opportunities then you will find motivation anywhere, anytime.