I find the following line of argument very convincing.
Edited, and with bold emphases added, from pages 47-51 of The God who weeps by Terryl and Fiona Givens, 2012:
The greatest virtue of the idea of premortal existence, undoubtedly, is in its solution to the problem of human freedom...
If our life carries hidden within its core our own eternal past, then we are free in a way no alternate model of human existence can account for.
[The most daunting problem of free will is the challenge to understand..]
how we can freely choose, if God made us - body and soul, mind and will, genes and instincts, predispositions and predilections, tastes and desires?
One can say, God created us and He created us free. But that just substitutes a declaration for an explanation.
No, if God is the sole author of all that is, then we cannot find our way clear to believe He is not responsible for our choices.
The ancients knew that something is free only if it is not caused or created by something else,
[...as JME McTaggart wrote]
If God created our souls, He 'could have prevented all sin by creating us with better natures and in more favourable surroundings... Hence we should not be responsible for our sins to God.'
This is the same logic by which we assign blame in all other instances where there is a creator and a thing created. If a bridge collapses, we hold responsible the person that designed the bridge or executed its construction...
But the fact is, as adults with moral awareness, we sense we are responsible for our own choices.
And the reason we know is we are, is because we feel guilt when we do something wrong...
The modern era has given us a dozen reasons to explain away those legitimate feelings of guilt we all experience...
But no rationalization can allay the insistent knowledge that we all confess in moments of secret honesty: we do wrong because we make a decision to do so, and feel guilty because we know we could have acted differently.
That means we had other options than the one we chose. If we could have acted differently, then we were free to act differently at that moment of choice.
Guilt, the legitimate remorse we feel for the deliberate decision to do wrong, is all the proof we need that arguments about determinism and predestination are a philosopher's game.
Guilt is how we know we are free to choose.
In our present, earthly form, we are clearly the product of forces outside our control that influence our personality, inform our character, and shape our wants and desires.
And yet we know we are free.
How can this be, unless there is something at the heart of our identity that was not shaped by environment, not inherited from our parents, and not even created by God?
Some scholars who thought deeply about the nature of sin came to the same conclusion that only pre-existence can explain human freedom.
It is no solution simply to insist God made us free.
Sin must mean accountability... Accountability must mean the freedom to choose.
And human freedom can only have its roots [to quote Julius Muller] 'in a sphere beyond the range of time, wherein alone pure and unconditioned self-determination is possible'.
The above seems to crystallize pretty much exactly my own feelings on this topic.
I agree that 'God created us and He created us free' is a pseudo-argument.
I also agree in rejecting the suggestion that free will is one of those things that humans cannot know, but in this matter we must simply submit to (what we imagine to be) God's will - because I find this to be not just an un-Christian, but an anti-Christian conception of the relationship between God and Man.
And I agree that pre-mortal eternal pre-existence solves the problem of free will in the way that nothing else does.
Therefore, the only remaining question is whether it is true that the human soul or spirit (of some kind) had a pre-mortal eternal existence (of some kind).
The idea of pre-mortal soul/ spirit existence is compatible with at least some authoritative, albeit unusual (some would say heretical - but that is to beg the question) views of Christianity including some Holy Fathers such as (apparently) Augustine and Origen.
There is also a great deal of indirect experiential subjective evidence implying pre-mortal existence;
and a strong metaphysical argument that if souls are eternal from mortality forwards, then this would tend to imply they are immortal from mortality backwards (i.e. if something exists eternally - as souls do, then it is hard to imagine a time when it was not existing, and was created from nothing. Easy to say this, but hard to imagine it);
and furthermore the alternative times suggested for when the human soul is created (conception, during embryonic development, birth etc) all seem to be arbitrary and implausible.
So, I conclude that it is true and the reality is that human souls eternally pre-existed mortality in some form and mortal life is (mostly) shielded from (full and explicit) knowledge of this by a veil of ignorance - such as to preserve the autonomy of mortality on the one hand; while, on the other hand, encouraging us with legitimate hope and sufficient understanding.