I have observed that one of the differences between great people and mediocres is that great minds will always get the job done regardless of how they are feeling.
Self discipline means doing what needs to be done regardless of how you feel about it. That is why you ought to get the work done; send that email, make the phone call, write that blog post, meet the client today (that way, you can meet another one tomorrow).
Just get it done! That is the hallmark of great men.