The question "how did the human nature of God the Son exist before Abraham?" does not follow from Jn 8:58 because Jesus said "I Am", not "my human nature exists". It is the divine Person Who eternally Is (God), and Who started to Be also human at some moment in 8-6 BC.
While the above is enough, I think I can enrich the coverage of this issue by relating it to the purely philosophical topic of the real distinction between essence and existence or act of being, as holding that real distinction (which afaik cannot be demonstrated from first principles) allows to provide, at the philosophical level, a description of the hypostatic union and an explanation of why Jesus' human nature is not a human person. The reasoning goes like this:
Christian doctrine of the Trinity states that each divine Person is the divine Essence. Those who also hold that in general there is a real distinction between essence and act of being must hold, per the doctrine of absolute divine simplicity, that the divine Essence is the Subsistent Act of Being, so that each divine Person is the Subsistent Act of Being. Therefore, the assumption of a human nature by a divine Person means that such human nature exists, from the moment of its creation, by the Subsistent Act of Being which that divine Person eternally Is.
In contrast, the position that the human nature of Jesus exists by its own contingent act of being gives rise to two serious problems:
A. Why would Jesus' human nature not be a human person?
B. Why did Jesus say "before Abraham came to be, I Am." (Jn 8:58) and not "before Abraham came to be, I Am in my divine nature."?
The only way to avoid these problems is to posit that in Jesus there is only one Act of Being, the eternal, Subsistent Act of Being of the Son. Therefore his human essence or nature does not exist by a created, contingent act of being, but by the Subsistent Act of Being of the Son. This case, in which a created essence does not exist by its own contingent act of being, shows that there is a real distinction between created essence and contingent existence.