How can God forgive my horrible sins?

Easily, because God is not like you. When you reflect on your sins, you think about forgiveness as you know it – how others have forgiven you, and how you have forgiven others. But God’s thoughts and ways are not like yours or mine. He forgives much faster and more fully than any person you know to a degree that is like how high the heavens are above the earth (Is 55:6-9).

Furthermore, your sins have already been paid for by Jesus Christ on the cross – He died for your sins! He suffered for what you did! God is satisfied with Jesus’ substitutionary death for you. Therefore, when you confess your sins, God is faithful and just to forgive you, because as the perfect Judge of heaven and earth, He does not allow double jeopardy for your sins.

If you want forgiveness, then confess your sins by name to God and repudiate them to Him (I John 1:9). Elihu suggested a good three-fold expression of confession (Job 33:27-28). God will forgive you (Ps 32:1-5). Use Psalm 51, David’s prayer of confession for his adultery and murder, as an example of how you should speak to God in your confession.

Do not compromise the important task of confessing your sins, or you will not prosper, and God will not hear your prayers (Pr 28:13; Ps 66:18). Godly sorrow and repentance that God accepts is very sincere and passionate (II Cor 7:10-11). You can clear yourself from any sin, if you will confess and repent. Ahab and Manasseh, the worst kings of Israel and Judah, respectively, found mercy with God for their confession and conduct of repentance (I Kgs 21:29; II Chr 33:11-13).

See More

For the glory of repentance and forgiveness … The R Factor.
For details about the forgiveness of sins … Forgiveness of Sins.
Jesus forgiving a very sinful woman … Forgiveness of a Great Sinner.