God is the great healer and forgiver. So if we seek his forgivness and are truly sorrowful over our actions, I believe He gives all of us a fresh start - including our purity.
My husband and I have mentored engaged couples for the past eight years and we do an evaluation with them called Prepare. For every couple who we have known to be sexually active prior to marriage, almost every one of them has low communication and conflict scores. So the issues that really need to be dealt with in the relationship are being dealt with underneath the covers and that's where they have a tendency to stay. The real things that need to be talked about and worked through never get dealt with - they just look prettier underneath the sheets. Eventually they will pop up and then they're not real pleasant to deal with.
That's what I believe adds to the chances for divorce - it's that a couple doesn't spend time working on the communication and conflict resolution skills when they're spending much of their free time in the bedroom.
God is pretty clear in scripture. No sex before marriage is found in His word - it's straight and simple from God's mouth to our ears. Sometimes we believe we know better for ourselves than He does... that's what gets us into trouble every time.
Here's some great scripture to read on God's design for purity -
It might make for an interesting date night together.
Romans 13:14
James 1:21
1 Thessalonians 4:3-6
Galatians 5:19-21
1 Corinthians 6:19-20
Ephesians 5:3
Proverbs 6:27 - this one is my favorite

"Can a man scoop fire into his lap without his clothes being burned?"
Leviticus 19:2
1 Peter 1:16
James 1:27
Isaiah 51:7
1 Peter 2:11
James 1:14
Matthew 5:28
Job 31:1
Hebrews 13:4
2 Timothy 2:22
Colossians 3:5-10
Romans 8:5-6
Isaiah 50:7
Consider another accountability partner as my guess would be your desire has a greater hold on you and she is giving in to you at times so she does not lose you. I believe that is a great fear of many woman. But, I can guarantee you something - God can and will give you the strength you need to overcome this. And... if you are the stronger person, he will build a great respect for you in this woman who loves you.
She will know beyond a shadow of a doubt that she will always be able to trust you sexually because you have sacrified so much to remain pure in your relationship. It's worth more than you know.
And remember - it's all about obedience to God's word.
Some other great books -
Sex Is Not the Problem (Lust Is) : Sexual Purity in a Lust-Saturated World
by Joshua Harris
Passion and Purity: Learning to Bring Your Love Life Under Christ's Control by Elisabeth Elliot