As I mentioned in an earlier post, it seems to me that to identify the three Persons of the Trinity as “Being”, “Known”, and “Willed” is to reduce three actual
Persons to abstract ideas. Akin to Socrates’ notion of the Forms, the ideas may describe the Persons of the Trinity, but how can the ideas be the Persons themselves? Should each Person be thought of as an ontological being, or simply as a modal relation within the Godhead?
Your objection is actually a very, very good one, and is the main one Aquinas is addressing when he lays out that real relations are actually distinct Persons. Wondering about this matter hardly makes you foolish, it actually shows that you have a keener grasp of philosophy than you may realize
That being said, the key to your objection is that the Essence of God is Personal, and the Essence of God is pure, actual being. If it were not, Modalism would be the logical result as you have indicated. I’ll do a “long” answer, and then a quick summary at the end.
Taking the question of persons and essence and God from another angle, let’s consider why when we think of our own essence it’s not a different person (and why when we think of others, our image of them is not a person in itself). First and foremost, our essence is not personal. This may seem strange since we are persons, but let’s consider this from a philosophical perspective. What this means is that while all the features of a person can be in place as an “essence”, it’s not “actualized” until created by God. The essence of a human person actually “pre-exists” in the mind of God, so to speak, but that doesn’t mean that the essence in question is eternal. We are “actualized” in time, because we change from being a “potential person” to an “actual person”, and change is a property of time. God chooses the moments in time to actualize this or that essence, and only when the essence is actualized is it truly a person; before that it’s the idea of the person (which is perhaps why God is said to know us for all time, even before our conception).
Now when we imagine someone, we are not dealing directly with their essence, but rather abstracting certain features of them and putting them together in our mind. Even if we WERE capable of fully comprehending the essence of a human person in our minds, it still wouldn’t be another person because the essence is not personal in itself, but is so only by being actualized by God. We’d be dealing with 99% of that person that we know, but we would forever be lacking the actualization of that person. For the Divine Essence this is different, however, because the Divine Essence is Personal by definition; it requires no outside actualization to go from potential to actual, and such an idea is utterly blasphemous because a ) it would mean there is something above God, and b ) it would mean that God was created at some point. Since the Divine Essence always is, it must be a Person since at the very least the Father is fully God, always is, and is a Person (of course the other Persons are too, but I stress the Father here because there’s no way in which the Father is said to have an origin).
Now, since God is omniscient, He can fully comprehend His own essence (which is something humans arguably can’t do, and we certainly can’t comprehend anything else’s essence, even that of a fly), and since that essence is Personal that means His “self-image” is Personal. If the Divine Essence were not personal, then God’s comprehension of it would be just like our own self-comprehension where we can abstract all the details except true personhood (our self-image doesn’t get up and walk around).
So it is a bit of a paradox, but the answer lies in the nature of the Divine Essence and how it is different from ours.
Summary:
The Divine Essence is Personal in itself, since the un-originate Father is Personal without needing any outside force to turn Him from a potential to an actual Person. This is unlike us as we need God to turn us from potential to actual. So when the Father “imagines” the Divine Essence, He “imagines” a fully actual person and not just an abstraction, since there is no limit to His power of Knowledge (we only have a limited self-image because our power of knowledge is not omniscient, and the object of our knowledge is not self-actualizing).