Paul wrote about this extensively and she must forgive you as Jesus forgives you. Here are some of my favorite scriptures when dealing with something like this.
I would talk to her openly and honestly about the problem you are facing. Do it in a non-confrontational manner, so I would recommend putting what you want to say like this, "(Name), God has laid this heavy burden on my heart about receiving your forgiveness about my past. I have sought counsel from Godly men and women on how to talk to you about this burden. These scriptures came up as a result of my counseling and I would like for you to listen to them because I feel that God wants you to hear this."
Colossians 3:12-14 Therefore, as God’s chosen people, holy and dearly loved, clothe yourselves with compassion, kindness, humility, gentleness and patience. Bear with each other and forgive one another if any of you has a grievance against someone. Forgive as the Lord forgave you. And over all these virtues put on love, which binds them all together in perfect unity.
Ephesians 4:29-32 Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen. And do not grieve the Holy Spirit of God, with whom you were sealed for the day of redemption. Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice. Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you.
Luke 17:3-4 So watch yourselves.“If your brother or sister sins against you, rebuke them; and if they repent, forgive them. Even if they sin against you seven times in a day and seven times come back to you saying ‘I repent,’ you must forgive them.â€
Mark 11:25 And when you stand praying, if you hold anything against anyone, forgive them, so that your Father in heaven may forgive you your sins.â€
Matthew 18:21-22 Then Peter came to Jesus and asked, “Lord, how many times shall I forgive my brother or sister who sins against me? Up to seven times?†Jesus answered, “I tell you, not seven times, but seventy-seven times.
There are more passages about how as Christians we are to forgive others as Christ forgave us for our sins. The moment you repented, you began the process of sanctification and to be like Christ. That is the key thing because you are a new creature in Christ and as Christians we are to accept you for who you are now not from when you were part of the world. I will pray for you and your girlfriend so you can both find peace.
Peace and blessings to you.