How can I be certain of God's forgiveness for me after my miscarriage, seeking His forgiveness, and repenting?

You have not committed a grave sin, but do not despair of God's mercy. The door of repentance is open, and God is the Oft-Forgiving, the Most Merciful, who accepts the repentance of the sincere and replaces their evil deeds with good ones. Repent to God with sincere repentance, increase in good deeds, observe your prayers, and have good expectations of God in His acceptance of your repentance and acts of worship.