We are born flesh, which is corruptible. That of course means the ability to be corrupted. This is why we can fall.
When we are born again, we are born of incorruptible seed. It cannot fall. The scripture says we cannot sin, because our seed remains in us.
1 Pet 1:22 Seeing ye have purified your souls in obeying the truth through the Spirit unto unfeigned love of the brethren, see that ye love one another with a pure heart fervently:
23 Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever.
24 For all flesh is as grass, and all the glory of man as the flower of grass. The grass withereth, and the flower thereof falleth away:
25 But the word of the Lord endureth for ever. And this is the word which by the gospel is preached unto you.
We were born of corruptible flesh, therefore we could fall. But when we are born again, we are washed clean (vs. 22) and married and joined to the Holy Spirit, becoming one spirit (1 Cor 6:17). Now we are born again of incorruptible seed that cannot sin and fall away in unbelief, as we are partakers of the divine nature. We are not our old man, and we are not God, but we are made from a union of our spirit and the Holy Spirit. We are a new creature. We are not God, we are individuals.
It is like a child, he is made of both his mother and his father, but he is neither, he is a new creation, and he is an individual. It is the same in the spiritual birth.
It is that part of us that is the Holy Spirit that prevents us from falling away in unbelief. It is the incorruptible seed that remains with us and abides with us forever.
Jhn 14:16 And I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever;
17 Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you.
No one had the indwelling Holy Spirit until Jesus was raised from the dead and glorified. But Jesus promised that he would send the Comforter (the Holy Spirit) who would abide with us FOR EVER, and would BE IN YOU.
This is why I believe it is impossible to lose salvation.
I agree there are many scriptures that on first glance appear to say a person can lose salvation, but I think they are speaking of falling away in unbelief before a person ever believes and is sealed forever by the Holy Spirit.
But the reason we could originally fall is because all we had was man's spirit, our natural spirit which is corruptible and could fall into unbelief.
I hope I have explained this well.