You are a sinner because you fall short of God’s standard and ideal for a man or woman set by Jesus’ perfect life; i.e, you fall short of His glory: You are born in sin, sin by nature, and sin by your own choice. Sin is anything you do against the nature of God that offends God: autonomy, unbelief, rebellion, irresponsibility. Going your own way and doing your own thing! (cf. Isaiah 53:6). You have not only done that which ought not to be done but have not done things you should’ve done. Any thought, word, deed, desire, or omission contrary to God is sin. It’s any want of or transgression of God’s perfect Law. Sin is literally “missing the mark,” or the goal.
But also, it’s in the heart where the seat of your thoughts arises and can have sins of omission as well as of commission (“For as a man thinks in his heart so is he.” “For out of the abundance of the heart flow evil thoughts….”)This means we sin by what we think and not just what we do. We look on women or men with lust and commit adultery in our hearts.
God doesn't justify the sin but the sinner: Even believers sin but they are justified (cf, Gal. 2:17) and God doesn’t count sins against them (cf 2 Cor 5:19). Jesus intercedes for us when we sin (cf. Heb 7:25; 1 John 2:1–2) and sympathizes about our weaknesses. We are still sinners, but justified ones and called saints, not sinners by God.
Sin is a virus that we inherited from Adam and we are in solidarity with him and have declared our independence from God in so doing. We are born the slaves of sin and cannot do anything but sin unless we are saved. But we are not made righteous but declared righteous or just. that’s why Christians still sin.
God has not already forgiven you unless you repent and believe in His Son. He is under no obligation to forgive you or it would be justice. But He forgives by mercy because of Jesus’ sacrifice on the cross. That is how much He loves you because His Son died for you. You believe by grace, you don’t conjure it up or do it on your own power, it’s the gift of God and He opens your heart and changes your heart from the inside out. It is by the power of God that you believe and repent, through grace as a work of God in your heart. He can turn your heart of stone into one of flesh.
It is God's job to convict you of sin (cf. John 16:8–13) and He says all have sinned and there is one righteous. That you are a sinner is an open and shut case. No one is perfect or good but God alone. the closer you bet to God, the more you realize and are convicted of your sins; you become more sensitive to the Holy Spirit. If you deny sin, read the Bible and realize the high bar that God sets and you will be enlightened to your sins. Satan accuses you, but God convicts you. There should be no doubt about it and you will feel guilt and shame and realize you are addicted to the power of sin and its slave.
You have forfeited your freedom by sin and can't quit by your own power. You are not a sinner because of some sin or that you happened to sin, but sin because you are a born sinner and it’s your nature. Just like you dont’ realize how much you're addicted to cigarettes until you try to quit, you must try to be good to realize how bad you are and that you cannot be good. But you must realize how bad you are to be good or to be saved. You will still sin but you will not want to and will have a penitent faith or believing repentance.
As far as believing it, you must read the Bible, hear the preaching of the Word, and confess your sins to God to have a lively sense of sin and forgiveness. The more awareness, the less sin. God convicts you by the Holy Spirit and that’s why He’s called that. I cannot convince you but God can by the ministry of the Spirit in your heart. Soli Deo Gloria!