What should a young man do concerning his daughter, especially with his ex-wife's refusal to adapt to him and her attempts to rekindle their relationship, and his fear of the impact of this on the child's upbringing?

You have done well by repenting and becoming upright. However, you erred in: marrying the woman before her womb was clear [of pregnancy], attributing the child to yourself, and marrying her without the permission of her guardian. The conclusion is that this girl is not attributed to you; rather, she is attributed to her mother. If the mother repents and wishes to marry you, and you are capable of being just between her and your other wife, then there is no impediment to marrying her to protect her chastity, and her daughter will be your stepdaughter.