I heard Rabbi Shimon Kessin say that when the time comes, one of the big surprises we are going to discover is that we are all of the same soul. Right now, we are only seemingly separated. To see someone, I have to leave my own realm, and look "out" and all I get is a vision of their externalities, but they are unreachable to me. In time, it won't be like that at all, I will instead get knowledge of them from "gaining knowledge" about myself, because the fact that we are separated won't be noticeable anymore.
What I am trying to illustrate with this is that
- The higher we go in the ladder to understanding Hashem, the more abstract and disconnected from human reason they become. I.e. to understand these things, it doesn't help to be smarter, more logical, or have bigger, broader knowledge. There certainly is a direct route from The Truth about how "our fear is apart, but still part of, but not a part of Hashem" makes perfect sense, to the "logical explanation" of how it makes sense, but ultimately that logical explanation is a hundred layers deep in "mashal", and can merely invite a person to try and climb higher.
- The way things are now, (especially) including the fact that there are "parts", is just a concealment. However, we have been very deliberately designed so we can't understand that. We can't see it any other way.
As a result of 1, you will end up with a lot of different explanations, some better, some worse, sometimes contradicting each other, some that appeal to you and some that make zero sense or sound heretical to you, even if sourced in Torah opinions you might otherwise deeply respect. Kabbalah does indeed confirm that we are זולתו, we are other to Him, but we also know that He has absolutely no help in making us whatsoever. We are working with a paradox, and ideas that we can't comprehend.
As a result of 2, you can see that free will is essential. This whole thing, that we are separate, is so we can have free will, and what is free will except our fear of heaven? This is what Chazal are teaching us with the statement you bring: Hashem's guiding hands of Hashgacha leave your "fear of heaven" alone. Hashem wants to ensure that you feel otherly enough to Him to be able to actually freely choose with your own being whether you fear and love Him or not.
The accomplishment of free will generally is one of those items in theology that are relegated to paradox, for reasons very related to your question of separateness vs oneness. That means the logical mechanics of how it can work are so beyond us we can't actually solve it at all, and are simply left with contradictions by logic. Yet we know from mesorah that we have it, even though it is a paradox, so we believe in it b'emunah. We also try to deepen our understanding despite this, in any way that we can.
Lubavitch Chassidus does have a lot to say on free will (for example), and this subject in general. It is heavily based on Rambam, what he writes in Mishneh Torah and Moreh Nevuchim. In this subject of Lubavitch ideas on free will, a key issue is that Hashem created us so that He could have us, and that having us has to be real, not forced. Which means, we have to consent and, furthermore, choose to be with Him wholeheartedly. To accomplish this, He has taken us to the lowest world, which is a world that has stripped away every last drop of force (including the true fact that there is no separation), so we are able to make a real choice to love and fear Him
The Lubavitcher Rebbe really had a strong innovation on this point (this answer will be based on this shiur), in that he taught us that the realm of a real Divine someone is beyond the paradox. It is a realm beyond logic itself, so when we talk about what's going on in that realm, we can only use metaphors.
The standard metaphor of free will is choosing between options using a rational brain. This is only a metaphor, and doesn't really make sense anyway (the rational "choice" is what it is, it's not something we can choose i.e. logic and reason are influential). But it teaches us that high up in our soul there is a real version of free will, which is Divine. It's not a choice between options, but, as far as we can say in words, it is choosing another someone, with one's own self, one's own someone-hood, or to put it in the original language, with one's essence. Take something superficial to you, external to you, and make it you.
The deeper mashal of free will in ruchnius is that we don't choose between options, but we choose who we are. This is closer to the real Divine version of free choice, because it's taking some thing, like a personality trait, or a subject, or a cause, and making it part of our identity. Choosing as in, making it yours. This is close but not quite the same as real free choice, because the closer to your real essence you go, the more removed we are from all "things", and all that there is is us, which is perfect because when you remove all things, which includes all external influences, then you are completely free to be you. We have the power on that level to wholeheartedly accept and make Him ours, or actually, make Him us, and become one by choice. Choice and self become one because, when there's no influence, then you are free to just be you. There's no option, but there's no force, it's true freedom, and true you.
The purpose of free choice, then, really is making some one else, you, by only your own choice (which is you) and nothing else, and doing away with all this "parts" business in the first place! Our essence and our choice are identical and this incredible power to become one like this comes from our Divine souls.
The appearance of separateness that exists in the world of Asiya will no longer be needed and will fall away to where we don't notice that anymore, and we will see only how we are one, and He is more infinitely One than we can have imagined, and there is no true separation between us, and hopefully by then, you will get an answer to your question!
For now it seems best we don't understand much about this yet, as it will make it less of a free choice to fear Him!
