We understand that you're going through a challenging situation, and we appreciate your heart's desire to forgive and move forward in Jesus' name. It's commendable that you value chastity and that the man you're considering for marriage has been honest about his past.
Firstly, let's address the issue of forgiveness. Ephesians 4:32 WEB says, "Be kind to one another, tenderhearted, forgiving each other, just as God also in Christ forgave you." As followers of Christ, we are called to forgive as we have been forgiven. Consider the Parable of the Unforgiving Servant in Matthew 18:21-35, where Jesus illustrates the importance of forgiveness.
However, it's also crucial to acknowledge your feelings and the struggle you're facing. It's natural to feel anxious about the situation, and it's important to process these feelings in a healthy way. Psalm 55:22 WEB reminds us, "Cast your burden on Yahweh, and he will sustain you. He will never allow the righteous to be moved."
Now, let's address the topic of sexual sin. It's important to acknowledge that sexual sin is serious, but it's also something that can be forgiven and redeemed. First, we are glad the man in your life is now a believer and his past was confessed and forgiven. 1 Corinthians 6:9-11 WEB reminds us, "Don't you know that the unrighteous will not inherit God's Kingdom? Don't be deceived. Neither the sexually immoral, nor idolaters, nor adulterers, nor male prostitutes, nor homosexuals, nor thieves, nor covetous, nor drunkards, nor slanderers, nor extortioners, will inherit God's Kingdom. Such were some of you, but you were washed. But you were sanctified. But you were justified in the name of the Lord Jesus, and in the Spirit of our God."
Regarding your anxiety and the imagery you're experiencing, consider 2 Corinthians 10:5 WEB, "Casting down imaginations, and every high thing that is exalted against the knowledge of God, and bringing every thought into captivity to the obedience of Christ." When these thoughts come, pray and ask God to help you take them captive and replace them with thoughts that honor Him.
Remember, marriage is a lifelong commitment, and it's important to enter into it with clear eyes and a peaceful heart. It is also important to not dwell on the previous sins of this man but rather to forgive as Christ has commanded. As a believer in Christ, we are called to forgive one another as Christ forgives. Therefore, we should pray for you to be able to truly forgive him and the others involved as well as pray for you to not dwell on his past sins. This will help you to move forward with this man if it is God's will to marry him.
Let's pray together:
Dear Heavenly Father,
We come before You in Jesus' name, lifting up our sister who is struggling with forgiveness and anxiety. Lord, we ask that You help her to truly forgive her future husband as you have forgiven her You have said in the Bible, For if ye forgive men their trespasses, your heavenly Father will also forgive you:
But if ye forgive not men their trespasses, neither will your Father forgive your trespasses. We know that this man's sins are forgiven by You, Lord, and we ask that You help our sister to forgive him and move forward in their relationship. Give her peace and clarity as she seeks Your will for her life and this potential marriage. Help her to cast down any imaginations that are exalted against the knowledge of God, and bring every thought into captivity to the obedience of Christ. In Jesus' name, we pray. Amen.
Lastly, continue to seek God's will for your life and this relationship. Trust that He will guide you and give you the strength and wisdom to make the right decisions. Be reminded that the Bible is clear that it is better to marry than burn with lust.
Prayer Focus: God, Thank You for loving me. God, I ask You in Jesus’ name please bless me with everything that I stand in need of and everything You want me to have. 
