One reason why these thoughts keep coming to mind is probably because you are trying to push them down and out of your mind without dealing with them. Sweeping them under the rug will allow them to keep coming back to bug you. As I see it, the solution is not to repress these thoughts but to deal with them by accepting them and lifting them up. Whenever you have these troubling thoughts, do three things.
First, if the relationship is heading toward marriage, do not be afraid to talk to your boyfriend or girlfriend about the struggle you are having. If you do not feel ready for this, perhaps you can speak with a priest or some other counselor you respect. However, remember that good relationships require open and honest communication. Also, it is better that these issues come to the surface before marriage instead of within marriage.
When you bring up your concerns, make sure not to blame him or her for the past but rather express the fact that you want to work through this issue together. Share your insecurities, fears, or hurts, and allow the other person to love you. This will require some vulnerability on your part and some patience and empathy from him or her. If your love is strong and forgiving, the two of you will be able to overcome this difficulty. In the meantime, do not fear that these haunting thoughts of the past will never diminish. Over the course of time, you should feel greater peace as your love deepens.
Second, do not dwell on the past. Instead, lift those thoughts to God when they come to mind. Here’s how: Give thanks that the Lord has brought your date away from that lifestyle. Also, you may not know this, but suffering has value when it is accepted and offered up to God. This means that you can offer up your suffering as a prayer for the person’s healing, for your date’s former partners, and for all those who are living in sin. Whenever the memories bother you, use that as a reminder to pray for these intentions. Because of Christ’s sufferings, our trials in life have redemptive value when we accept them with faith and offer them up to him. In other words, Jesus can use your suffering to bring grace to others. You need to realize the good that the Lord can do through it.
Finally, resolve never to lead your date into sin. The Lord will heal you both as he knows best. In the meantime, let the pain become a prayer.