How to stay motivated for grant applications
Depending on your field, and the kind of postdocs you are applying for, 4-5 days might not be enough time (for European fellowships such as Marie Curie, for example, I would spend maybe 2 weeks).

But as far as maintaining motivation, it is not easy. You have to keep in mind that there are a LOT of other bright people applying for these, and an important factor is always luck (who is chosen to review your application, and what specific things those reviewers choose to emphasize).

Best advice: Don't give up.  Keep applying, at some point you will maybe get lucky. Until then, keep publishing, keep working. And remember, these big postdoc fellowships are not necessarily required to be a successful academic.