One reason I wanted to reprint your letter is because I hope God will use it to wake up someone who has been deceived into thinking the devil's way is better than Christ's. It simply isn't true (as you have discovered), and nothing is worse than looking back over your life and realizing you have wasted it or worse.
The past can't be changed, much as you might wish it could. But listen: The past can be forgiven, and God can give you hope for the future! The Bible tells us that God loves us even when we don't love Him. There is much truth in the old saying you may have heard: God hates the sin but He still loves the sinner. And God still loves you; He loves you so much that He sent His only Son into the world to give His life for you, so you could be with Him in Heaven forever.
No matter how great your sin, God's grace is greater. The Bible says, "God demonstrates his own love for us in this: While we were still sinners, Christ died for us" (Romans 5:8). Turn to Christ and confess that you are a sinner and need His forgiveness. Then ask Him to come into your life and save you and He will.
As a footnote, don't assume your arthritis is a punishment from God. When we know Christ, we know He is always with us even in the midst of pain and weakness and our hope is in Him.