The only sin God can't forgive is the sin of refusing His forgiveness.
God offers us forgiveness as a free gift free because Christ paid for it by His death on the cross. But like any other gift, you must accept it and make it your own. The Bible says, "The wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord" (Romans 6:23).
What you did was very wrong, and I can't pretend otherwise or gloss over it. Furthermore, when we break God's law there are always consequences and your guilt and misery are part of them. That, incidentally, is one reason I wanted to reprint your letter, because someone may be reading this who is being tempted to take the same path you have, and your experience should be a warning to them.
The first step you need to take is to turn to God and seek His forgiveness. God loves you and wants to lift your burden of guilt and shame and He will, as you open your heart to Jesus Christ. Don't let another day go by without Him. Then ask Him to help you rebuild your marriage, with Him at its center.
Then get the medical tests you need to find out if you have been infected with a sexually transmitted disease. This is a serious matter, and you must not delay any longer.