"Forgiving yourself" may not be the best advice for all procrastinators, but I think the idea is that when many people procrastinate, it causes them extra stress when the decision comes back to bite them. A part of that stress is simply because they have less time to do what they need, but another component is that they are kicking themselves for being stupid in the past. Forgiving yourself lets you remove the stress from the latter part, and thus would reduce the total amount.