Michael, are you and your girlfriend Christians?
If so, you will surely know that Jesus took all our sins upon Him when He died on the cross and by His death we can receive forgiveness.
You might like to read more about forgiveness in this
article by Randy Alcorn.
Here is a clip from that article:
You may feel, "But I don't deserve forgiveness after all I've done." That's exactly right. None of us deserves forgiveness. If we deserved it, we wouldn't need it. That's the point of grace. Christ got what we deserved on the cross, so we could get what we don't deserve—forgiveness, a clean slate, a fresh start. Once forgiven we can look forward to spending eternity in heaven with Christ and our spiritual family (John 14:1-3; Revelation 20:11-22:6).
God does not want you to go through the rest of your life punishing yourself for your sins. Jesus said to a woman who had lived an immoral lifestyle, "Your sins are forgiven. Your faith has saved you; go in peace" (Luke 7:47-50). Jesus was surrounded by people who were rejected by society, but who found compassion, forgiveness and hope in his love.
No matter what you have done, there is no sin beyond the reach of God's grace. He knows everything, so no sin we have done can surprise him. God has seen us at our worst and still loves us. The apostle Paul was a murderer—he had participated in the killing of Christians. He called himself the "worst of all sinners" (1 Timothy 1:15-16). Yet God not only forgave him, he elevated him to leadership in the church. There are no limits to the forgiving grace of God.
Having trusted God to forgive us, we must resist the temptation to wallow in our guilt, for in fact we are no longer guilty. (The beautiful thing about God's grace is that it isn't based on pretending we didn't do something wrong, but realizing that even though we did, we are now fully forgiven.)
If you do not happen to be believers in Christ, then that is something that you might like to talk about more on here too.
You might believe you deserve only the very worst possible punishment and that you cannot be forgiven at all.
But in spite of such feelings, it is not true that God cannot forgive you.
Forgiveness and healing is offered to all of us from our Father, and you do not have to punish yourselves further if you accept that Christ is your Saviour and you turn to Him with true repentance.
There is quite possibly something else that you may need to deal with.
True repentance means turning away from sin. If you find yourself unable to accept God's forgiveness, it may be that you need to look a little closer at repentance and what that means in your situation. Are you still feeling guilty because you are still knowingly breaking God's laws and living in sin? Prayerfully search your hearts and your consciences. Consider your relationship with each other in the light of your relationship with God.