Hi Tellie, I hope you are absolutely fine. You are believer of Jesus Christ and I am a Muslim and I shall try to narrate all what I know to help you.
Sex with a person who is not your life partner (spous) is absolutely wrong and is considered as a Major Sin; not only as per teachings of our religion Islam and our Holy Book Quran but it is also absolutely wrong as per teachings of the Holy Bible. I don't know whether you have read the Holy Bible from start to end but I have read Old and New testaments and have compared it with out Holy Quran and let me tell you the fact, Sex is considered a Sin as per the teachings of the Holy Bible.
God has created the relations i.e. Father, Mother, Brother, Sister, Son, Daughter..... and has created all kind of feelings. The feeling of Hunger, the feeling of Thurst, the feeling of Pain, the feeling of loneliness..... and like all other feelings, God has created sexual desires. You feel sexual desires and it means you are a normal human. And for fulfillment of your sexual desires, God has created a relation of a Life Partner, a Spouse; a relation of Husband & Wife.
God allows every human to fulfill his/ her sexual desires with the person whom God has created with a correct relationship. A life partner with whom you get married, with whom you spend your life, with whom you plan your future and with whom you enjoy the innocent talks of your kids, is no doubt your actual and correct life partner with whom you share each and everything, even with whom you can share your bed.
Secondly, as far as your guilt regarding this sin is concerned, it shows your pure soul and it means you have power to resist against this sin. So, no matter how big your sin is, just imagine how big your God is.
Even when I realize how many sins I have done, I just consider them so high than the mountains, so big than the oceans and it really make me upset.... BUT when I realize that these all sins are not bigger than the blessings of my Allah, not bigger than the favours of my Allah then I just get relaxed and I ask forgivenes from my Lord.
So, I sincerely advise you to stay away from this sin and if you think you can't stay away from this anymore then think about to get married. Also, ask forgiveness from God and I believe He will definitely forgive your past sins.