"Is the ego’s biggest fear loss?"
Mainly the ego is afraid of the truth and what the truth implies. The Holy Spirit's Answer to the separation is that it hasn't happened at all. He is aware that the split mind isn't real, and therefore the ego itself is an illusion. Everything true about the truth automatically renders the ego nonexistent.
The ego therefore sees the Holy Spirit - or the truth - as a threat to its existence, suggesting it does not exist at all, or that it is already dead.
In the world, it seems that to be afraid is a mixture of many different reasons and justifications. It takes on all forms and pretends to be about something external. It could be fear of judgement or loss or of having something taken away. It could be fear of punishment.
But really these are in a way, artificial fears, because they are a disguise that covers up an ambition to be afraid on purpose. Although it looks on the surface as though the ego mind is afraid because of various things external to it, underneath those fears is a motive to use fear as a weapon to produce a result. Fear is not the relinquishing of control but an attempt to control further.
These make-believe fears are not real fears. There are effectively no real fears. They are lies that the ego believes are true. It doesn't really believe that a fear of loss is major problem. It doesn't even perceive that a fear of death is a major problem. It knows deep down that its use of fear is two-sided and two-faced, and that it is a way to bring forth that which it wants overall, which is the undoing of God's Creation.
What really appears to threaten the ego is Reality. It implies in every way possible that everything about the ego is false and untrue. To the ego itself this comes across as abuse, offense, and attack. The ego feels ignored by the truth and shut out from it.
It cannot participate in it and sees it as an enemy. It wants to get away from it as though, to it, the truth is some kind of evil come to destroy the ego self. The ego can do nothing to stop it or change it and this threatens it greatly.
Since the truth is love, you could therefore conclude that the ego's biggest fear is love. It was made to bring doubt and question to what is certain and to undermine what love is. It is the opposite of love, and so love is its opponent.
It makes no real sense to be afraid of love because if you were clearly tuned to perceiving love as it really is, you would experience only joy and celebration. But when love is perceived through the lens or distortion that is the ego, it comes across as far from cause for celebration. Instead it is interpreted through a mistaken perception which causes the ego to perceive threat, danger and attack where there is none at all.
This is because the ego sees in the clear mirror of love a reflection of its own belief that attack is real and death is inevitable. It sees reflected back to itself its own self-attack and its own denial of Self. So you could indeed say that the ego is afraid of its own reflection.
Recognize that the ego by itself is nothing at all and is simply a mechanism for seeing in a distorted way - of seeing what isn't really there. It is an hallucinatory drug for the mind. It has no reality of its own, and by itself it cannot perceive. It isn't a living thing at all. It is like a tool which you can pick up and use and put down again.
It requires for you as a being who has awareness and the ability to be experiential, to take ahold of this device and pick it up and put it over your awareness, to distort perception. Once you do this, you begin to look through it and perceive what it would show you. It will filter the world around you and displace the truth away from where it is.
What you end up with is an apparent experience of "being the ego" as though it and your Self have become united, or at One. Joining with the ego mind you turn your sense of identity into it and pretend to live it. This appears to give life to the ego or makes it seem like it has a life of its own. You empower it and your belief in it seems to bring it to life, like a dead puppet on a string which you are manipulating.
When you do this and have aligned yourself with its sense of perception, you will see yourself incorrectly and will misperceive through its eyes. And what you will see will be an interpretation of the truth which is distorted. This will show you an image of a false God, which is not really there, who seems to threaten your very existence.
You will think of God, or truth, as through you see in it a vicious intent and an intent to destroy the Son of God. This is your own attempt to destroy yourself reflected back to you, because what you project out comes back to you as perception. To this you react in great terror, because through the eyes of the ego God means you do not exist.
In God your ego cannot see a place where it begins or ends, and it has no definition. It is meaningless in the light of truth. This terrifies it greatly and is the sole stimulation for justifying putting up defenses against the truth to shut out reality. This of course creates blocks to awareness, which produces all states of sickness as defenses against the truth.
You reason it makes sense in the interests of self-preservation to protect your "self" from the attack of truth upon the ego, having identified yourself as being the ego yourself. And so in trying to defend yourself against Life you choose death instead, as though death can help to preserve your existence. This is the backwards logic of the ego mind.
To use the ego in this way is to use fear, automatically. And fear will be your experience, because the ego is a fear-producing device. It must produce fear because what it sees is not God's truth. It is afraid of what its own mind concludes about itself.
Ultimately all fear is the result of your own belief that you yourself deserve to die as an escape from the wrath of God, which you perceive is coming to enact revenge for your vicious sin of separation. It is not just that you are afraid of what God will do, but you are afraid of what you will do to yourself.
Fear isn't real. And along with it, the ego isn't real either. And this it fears the most. When you decide to no longer identify your sense of Self with it, its way of seeing fearfully will no longer be joined with your way of perceiving, and you will no longer have a reason to defend it against the truth. The truth has come to set you free and you can return to everlasting happiness.