Commentary on Meditations: B7:16

The directing mind does not disturb itself: for example, it does not frighten itself or lead itself to desire. If anyone else can frighten it or cause it pain, let him do so: of itself, of its own judgement, it will not deliberately turn to such modes. The body should take care, as far as it can, to avoid harm; the sensual soul, which feels fear or pain, should say if it does so; but that which makes general assessment of all these things will not suffer at all - it will not itself rush to any such judgement. Of itself the directing mind is without needs, unless it creates a need for itself: in the same way it is untroubled and unhindered, unless it troubles or hinders itself.

There is a part of our soul that is truly "in charge" of our attitude and desires and fear.  Nothing can actually touch that part of our soul.  Words spoken, pain administered, a jolt of adrenaline - nothing can truly touch that part of our soul.  That part of our soul is independent.  It is what makes us sovereign.

The absolute crucial part of Marcus' meditation here is that the part of our soul that is independent and sovereign (or the directing mind), "is without needs, unless it creates a need for itself."  The body has needs (water, food, air).  But this directing mind of ours, does not have any needs, unless it says so.  I speak as though "it" is a third person, but in fact, "it" is the unique you.  "It" is what makes you, you.  The directing mind of our's, is our Inner Citadel.

I believe I've cited this passage previously, but I will do so again.  This is from Hadot's The Inner Citadel p. 106-107:

In order to understand what Marcus Aurelius means when he says that things cannot touch the soul and are outside of us, we must bear in mind that the word "soul" could have two meanings for the Stoics. In the first place, it was a reality made of air (pneuma) which animates our body and receives the impressions, or phantasiai, from exterior objects. This is often what Marcus means by "soul." Here, however, when he speaks about "us" and about the soul, he is thinking of that superior or guiding part of the soul which the Stoics called the hegemonikon. It alone is free, because it alone can give or refuse its assent to that inner discourse which enunciates what the object is which is represented by a given phantasia. This borderline which objects cannot cross, this inviolable stronghold of freedom, is the limit of what I shall refer to as the "inner citadel." Things cannot penetrate into this citadel: that is, they cannot produce the discourse which we develop about things, or the interpretation which we give of the world and its events. As Marcus says, the things outside of us "stay still"; they "do not come to us"; rather, in a way, "it is we who go toward them" (XI, II).

(see also Citadel p. 41, 113, 268)