I've been have a really hard time dealing with my past and my relationship. My ex-boyfriend of several years and I broke up. I lost sight of the Lord and I sinned. I had sex with other people, and I am full of so much guilt and shame. My ex and I are working on getting back together. He knows my past, but he never had sex with anyone else because he loves me, which makes me feel disgusted with myself because my past is not who I am at all. Now we are working to follow God and repent. I still have so much guilt. Every night I think about it and how much I wish I could take it back. I feel like it's creating so many barriers against reaching our full potential in our relationship because it obviously is not something that doesn't bother him. I'm afraid because I had sex with more than one person I will not be accepted into his kingdom. I also feel like there's no hope and I ruined my relationship with my boyfriend. I just feel like giving up.
There is a fundamental flaw running through your note. God never said that it was a sin to have sex with more than one person. "Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for fornicators and adulterers God will judge" (Hebrews 13:4). "Fornication" is having sex with someone you are not married to. It is possible that you meant that your boyfriend is a virgin, but since you said, "he never had sex with anyone else," it leads to the conclusion that you and he had sex. If that is correct, then the sin began with the two of you and you continued the same sin with other people after you broke up.
I'm glad you are interested in repenting, but repenting does not mean returning to having sex with only your boyfriend. Repentance is giving up sin, which in this case means no longer having sex with anyone but your future husband after you and he marry.
All sins can be forgiven. To claim otherwise is to say that God lied when He said, "If we confess our sins, He is faithful and righteous to forgive us our sins and to cleanse us from all unrighteousness" (I John 1:9). You can spend your time in worldly sorrow or you can pick yourself up and live as God wants you to live. "For the sorrow that is according to the will of God produces a repentance without regret, leading to salvation, but the sorrow of the world produces death. For behold what earnestness this very thing, this godly sorrow, has produced in you: what vindication of yourselves, what indignation, what fear, what longing, what zeal, what avenging of wrong! In everything you demonstrated yourselves to be innocent in the matter" (II Corinthians 7:10-11).
Feelings don't alter the facts. Your feelings don't create an alternative reality. Listen to God and stop listening to your feelings.