YRNBU ? this is far more complicated than that. I feel for you, OP, I really do.
It sounds like you made decisions on the information you had at the time: i.e. you decided to have a sexual relationship with DP because your priest, at the time, said he would not marry you. You had to make a realistic choice at the time. If he hadn?t been so rigid and devoid of true Christian compassion you?d probably be married and not in this position now.
From what I remember from my Christian days, you don?t really need to atone for you sins as such because Christ died for you on the cross and took the sin away for you, if you truly believe in Him.
But it could make your relationship stronger ? if DP feels ok with it. I have heard of couples abstaining for a time before they get married even if they are already sexually active, and how it improved their sex life once they were married: absence makes the heart grow fonder and all.
But how are you going to spend your time with him over the next 12 months? When you switch the lights out at night are you just going to roll over and go to sleep, or if you want to develop together spiritually, will he be happy to pray with you?..
Are you willing to compromise with DP over this? What if he says ok to no penetration but what about for play, kissing etc?where are you happy to draw the line?
If you really look in your heart of hearts and there is no sign of depression, then, all I can say is don?t confuse guilt with what God?s asking you to do. Nothing will crush you?re faith faster.
Sounds like you?re going through a transition with your beliefs and I wish you all the best.