The further I get into Stoicism, the more the second part of this resonates. I'm not big into motivational phrases, but there are parts of this that really help me regain focus. As with a lot of Epictetus, it's confrontational and somewhat condescending.
How long will you then still defer thinking yourself worthy of the best things, and in no matter transgressing the distinctive reason? Have you accepted the theorems (rules), which it was your duty to agree to, and have you agreed to them?
Why are you still delaying from following the teachings that you have accepted will make you happy. How long will you then still defer thinking yourself worthy of the best of things?
What teacher, then, do you still expect that you defer to him the correction of yourself? You are no longer a youth, but already a full-grown man. If then you are negligent and slothful, and are continually making procrastination after procrastination, and proposal (intention) after proposal, and fixing day after day, after which you will attend to yourself, you will not know that you are not making improvement, but you will continue ignorant (uninstructed) both while you live and till you die.
Are you waiting for some new teacher to come and fix you? You're an adult; act like it. If you procrastinate and put off personal growth, you will never improve.
And now, my favourite part (possibly of the whole book):
Immediately, then, think it right to live as a full-grown man, and one who is making proficiency, and let everything which appears to you to be the best be to you a law which must not be transgressed. And if anything laborious, or pleasant or glorious or inglorious be presented to you, remember that now is the contest, now are the Olympic games, and they cannot be deferred; and that it depends on one defeat and one giving way that progress is either lost or maintained.
This gives me shivers. As covered in part 50, treat those things which you have decided are good as inviolable laws. When you are tempted to break them remember: this moment is the contest for your improvement; it cannot be delayed. Progress is made and lost through every choice we make.
Socrates in this way becomes perfect, in all things improving himself, attending to nothing except to reason. But you, though you are not yet a Socrates, ought to live as one who wishes to be a Socrates.
The Stoics have a lot of admiration for Socrates and often hold him up as the ideal Stoic. The thing I love about this closing phrase is that makes clear that no one is Socrates; none of us have perfected our Stoic practice. But if we want to move towards that goal, we should live in a way that moves us in that direction.