How do we know that God's mercy endures forever?
Because Jesus Christ died on the Cross for our sins.
He became sin that we would be made the righteousness of God in Christ (2 Corinthians 5: 21)
He became cursed that we would be blessed (Galatians 3: 13)
He died for all our sins:
"And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses;" (Colossians 2: 13)
We praise Him for paying for our sins, and His payment enacts the New Covenant:
"For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:
11And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest.
12For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more."
Rest in His forgiveness, and praise Him for paying for everything!