There is no first birth, or incarnation. In Brahma Sutras 2.1.35-36 (Swami Vireswarananda translator) it says:
If it be said (that is) not (possible) for want of any distinction in work (before creation), (we say) no, because of (the world) being without a beginning.
And (that the world is without a beginning is reasonable and is also seen (from the scriptures).
And Sankara’s commentary on verse 35:
…is answered by the Sutra, which says that creation is without a beginning and the question of first creation cannot arise. It is like a seed and its sprout. So the individual souls have always had a previous existence and done good or bad deeds in accordance with which their lot in a subsequent creation is ordained by the Lord.
And his commentary on verse 36:
Reason tells us that creation must be without a beginning. For if the world did not exist in a potential state in the form of Samskaras (impressions), then an absolutely non-existing thing would be produced at creation. In that case even liberated souls might be reborn. Moreover people would be enjoying or suffering without having done anything to deserve it—an instance of an effect without a cause, which is absurd. It cannot be attributed to primeval ignorance, which, being one, requires the diversity of individual past work to produce varied results. The scriptures also posit the existence of the world in former cycles in texts like ‘The Lord devised the sun and moon as before’ (Rg-Veda 10.190.3).
So partiality and cruelty cannot be imputed to the Lord.
Your soul is eternal, it has always had a previous existence, it never had a first birth.