I say that I sin because I still do things that are against, or opposing to, God. Such as, He has said that we should not lie. If I choose to tell a lie (even a "little" one), then I have gone against Him, and have sinned. Making a mistake is not necessarily a sin.
Christ died, was buried, and was resurrected on the third day to pay for the "wages" of (or price of, or punishment for), sin. The penalty for sinning, or going against God, is Hell.
No, sin is not gone because I still live here on earth in my "flesh" body. That part of me is still capable of going against God. However, when I believed and trusted in Jesus and His finished work on the cross, I was given a pardon from that penalty. ALL the sins I did do, and will do, was forgiven. I can now live in freedom with the knowledge that I get to be with God and Jesus in Heaven when I die, and not be in Hell.
This, of course, leads to the question: So, does that mean I can just sin all I want too now that my penalty has been taken from me?
NO! LOL I don't want to do those things (not that I can't or won't), that are against my Heavenly Father. I want to do better and better because of the love He showered on me. I want to read His words and learn more about Him, so that I can be more like Him. But, praise be to God, when I do screw up, I don't have to worry that I am going to be d*mded, or that I am going to struck down, or that someone I love is going to suffer because of something I did wrong.
I must say, the Apostle Paul talks about this soooo much better than I can. LOL I would gently suggest reading his letters to perhaps get a better insight. AND, don't be afraid of asking lots and lots of questions!