Hi Apple! I'm sorry you are in this position. As a disclaimer, I have never been in a situation like yours, so I may not be qualified to speak on this. I can only try to understand your experience through my own experience. That being said:
Thoughts on cheating:
The Church is the bride of Christ. Love for God is keeping his commands. When we sin, we follow an idol. I think sin harms our relationship with God in the same way cheating harms our relationship with people we love.
Thoughts on repentance:
I am a sinner. I am unfaithful in my relationship with God and am convicted by the Holy Spirit to repent of my sins. I believe that true repentance includes not just a confession of sin, but also a turn from it. And Hallelujah! God is faithful and just and forgives us our sins and purifies us before Him from all unrighteousness. He does not tempt us beyond what we can bear, so that sin becomes all the more detestable as it becomes an active choice.
What I would do:
If I were in your position, I'd imagine I would feel betrayed, confused, hurt, upset, and angry. I would ask myself why he repented. I would wonder if he was sincere. You mentioned that he apologized and stopped his behavior. Great! I would have a conversation with him in love, not in suspicion, to discuss what we can do to prevent this from happening again and decide on active changes. I would ask myself if I have forgiven him and if reconciliation is possible or has been reached yet. Most importantly, I would pray to God asking for wisdom in the situation and for a filling of the Holy Spirit in both of our lives to guide us in love to whatever He has in plan for us.
I would like to remind you that your partner is not the enemy. The person he was unfaithful to you with is not the enemy. Our struggle is not against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness and against the spiritual forces of evil in the heavenly realms. So don't think for one second that the two of you relying on your own strength will ever be able to overcome evil. However, hallelujah! There is hope for the future so that we do not need to be afraid. Do not be anxious about anything, but in every situation, by prayer and petition, with thanksgiving, present your requests to God. And do not worry! Worrying never added a single hour to anyone's life. God holds all power and loves you, so you can cast all your anxiety on him because he cares for you. And we know that in all things God works for the good of those who love Him, who have been called according to His purpose. Even Lazarus's sickness was for God's glory so that Jesus could be glorified through it.
Lastly, I would just like to thank you for sharing and allowing me to respond. I really needed to be reminded of God's Word while typing this, and I hope that someone else might be reminded as I was.
4
This is a good place to connect with other believers and share our thoughts. Re your situation, it's good you're not married yet. You shouldn't consider marriage with this person until there is no sign of interest in other women. The fact that he's a Christian and doing this doesn't make him look sincere, and as the Scripture says 'But I say, anyone who even looks at a woman with lust has already committed adultery with her in his heart' Mt 5:28. 
