https://www.amazon.com/dp/0764215663/ref=cm_sw_r_cp_awdb_imm_t1_.TX9FbT13MCJ9
Buy this book as a good start.
It’s never too late! God loves you, and will keep loving you, regardless of what you have done. We all do things that we aren’t particularly proud of. Things that we regret, that we feel ashamed of. It is never too late to seek forgiveness, repent, and change your behaviour/circumstances going forward.
I’m not sure if it’ll help, but you may find this book a useful resource: https://www.amazon.com/Pure-Desire-Triumph-Others-Temptation/dp/0764215663
It’s written with men in mind, but I think it’s just as relevant for women.
I was in a relationship once and felt ashamed and guilty about having sex with my boyfriend at the time (just like you, I lived with my boyfriend - we were together for nearly 7 years!). I knew it was wrong, but it seemed impossible to continue my relationship with him without it. Eventually I worked up the courage to leave that relationship.
Now I’m married to a wonderful Christian man, and sex with him is how God meant for it to be - between a man and a woman, who are married to each other. We have a much better relationship than I could have dreamed of, have the same values and goals, and have plenty in common because of our shared faith.
As Christians, we aren’t supposed to date or marry non-Christians, and for good reason. It would be awfully difficult (not to mention, sad) to be in a relationship with someone who is going to hell unless they repent and accept Jesus as their Lord and Saviour. I think it would be difficult to also share the same values and vision in life - many non-Christians suffer from idolatry, whereas we know our God is a jealous God who is very much against idolatry.
As much as it might be difficult, and not what you want to hear, I think you should break up with this person. You shouldn’t be in a relationship with someone hoping that they will change and become a Christian. If you are truly meant to be together, he will become a Christian and you could get back together then if that’s what you both want to do. Continuing with the status quo seems untenable, and is likely to result in things getting worse - not better.