Your honest and broken confession has been heard, and I thank you for speaking plainly what many hide in darkness. You say you feel alone, yet in this very struggle you are not alone. Many of the Lord's people have walked this painful way, cast down by wandering eyes and lustful thoughts, and tempted to believe that their sin has put them beyond mercy.
You have cursed God and blamed Him though you know the fault is not His. Do not turn away from this truth, for faith and repentance are twins born together. The faith that truly trusts Christ is never a faith that thinks lightly of sin. If we have sinned with our lips and with our hearts, let us own it before Him without excuse, for He is ready to forgive. The essence of faith is not merely to know that Christ is strong to save, but to drop down into His arms and trust Him to save you even now. He delights in such faith, for faith gives Him a hearty welcome, and Christ highly esteems the sinner who believes in Him.
You are weary of lustful thoughts and of falling into that unclean snare which defiles the mind and wounds your marriage. Know this, that the perseverance which God works in His saints is not by the removal of temptation, but perseverance in holiness. He has said, they shall not depart from me, and He keeps alive the fear of God in our hearts by the hearing of His Word and by watchfulness and prayer. Your marriage covenant with your wife, one man with one woman for life, is honourable before God, and all that draws you away from purity within it is not to be excused but to be fought against with the shield of faith. Faith can quench the fiery darts, whether they come from Satan, from the world, or from your own wicked heart. This is the victory that overcomes the world, even our faith.
You say you want to give up and follow Christ no more. Hear the gentle censure of the Saviour to little faith, O you of little faith, why did you doubt. Why wavering faith in an unwavering promise. Little faith may yet increase exceedingly until it ripens into full assurance. Christ loves great faith and He delights to see it, but He does not cast away little faith. The marriage between your soul and Christ, if it be true, can never be dissolved. The purpose of God secures it, and the effective redemption which Christ has worked out secures it, and the Holy Spirit abiding with us forever secures it. Though you are feeble as a lamb, the promise stands, they shall never perish. He that endures to the end shall be saved, not because his own strength holds him, but because God preserves him and keeps his way.
Therefore do not give up. Turn again to the Lord Jesus Christ, for there is salvation in no other. Confess your sin to Him, seek His cleansing blood, and ask Him for a new heart that loves purity. Be diligent in hearing the Word, for faith comes by hearing. Flee from that which feeds lust, set a guard before your eyes, seek godly fellowship and accountability, and pray without ceasing. When you fall, rise again and look to Christ.
We will pray for you, that the Lord would have mercy upon you, forgive your hard words against Him, cleanse you from all uncleanness, strengthen you to love your beautiful wife and children with a pure and faithful love, and keep you by His power until the day of Christ. He is able to make the righteous hold on his way, and he that has clean hands shall wax stronger and stronger. Look to Him and live.