You have asked a painful and honest question that many have wrestled with when the weight of deliberate sin feels too heavy to bring to God. The guilt you describe, the sense that your sin is ever before you and that you cannot hide it from yourself, is exactly what Scripture describes. When you try to cover sin and hold it inside, God's hand feels heavy day and night and the heart becomes dry and troubled. That heaviness is not a sign that you are beyond mercy. It is the conviction that is meant to bring you to confession.
The Bible makes a distinction that helps here. To sin can mean to miss the mark, and that can happen through ignorance or weakness, even when you do not fully understand what you are doing. Jesus prayed for those who crucified Him, Father forgive them for they do not know what they do, because they needed forgiveness even for sin done in ignorance. But what you are describing, and what you name as lust done deliberately for pleasure even after you knew better, is what Scripture calls transgression. That is willful, defiant rebellion against the law of God. It is more than a mistake. It is a knowing choice to do what God has said not to do.
King David is an example of this. His act with Bathsheba was not an accident. It was willful. Then to hide it he tried to cover his guilt, first by bringing Uriah home from the battle and then by conspiring to have him killed so no one would know. He thought he had covered it, but God saw it and sent the prophet Nathan to expose it. David finally said, I have sinned against the Lord. For a long time he had carried that unconfessed sin and was troubled in his conscience, but when he stopped covering and confessed, he received the word of forgiveness.
That pattern is given for you. Proverbs says he who seeks to cover his sin shall not prosper, but whoever confesses and forsakes his sin shall find mercy. There is no forgiveness in justifying your actions or in trying to hide them, even from yourself. Forgiveness is as close as confession. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. David wrote about this after he was forgiven, Oh how happy is the one whose transgression is forgiven, whose sin is covered, to whom the Lord does not count iniquity. He had known the misery of guilt, then he said, I acknowledged my sin to You, and You forgave the iniquity of my sin. That happiness only came after confession.
You ask why you should accept mercy and grace if you sinned willfully. The answer is not that willful sin does not matter, or that grace gives you a license to sin. It does matter, and you cannot sin with impunity and expect God to simply overlook it. Some have misused grace to say let us sin freely so grace may abound, and Paul answers that with, God forbid. How can we who died to sin live any longer in it. Where sin abounded, grace did much more abound, but that grace is not permission to keep practicing sin. Scripture warns to keep yourself from sin and to flee youthful lusts which destroy the soul, and it lists among the works of the flesh adultery and fornication and impurity. The lust of the flesh and the lust of the eyes and the pride of life are of the world, not of God, and the world is passing away with its lusts. To practice sin as a way of life, to live in it deliberately, is to walk in lawlessness.
Yet it is the glory of God to pardon sin. He says, Come now let us reason together, though your sins are as scarlet they shall be as white as snow. He will abundantly pardon, but that pardon comes on His terms. Justice is getting what you deserve, and if you will not repent and confess you will receive justice. Pardon comes when you turn from your sin, acknowledge your guilt before God, and believe that God has provided the basis for forgiveness by laying your sin on His Son. In the old offerings a person would lay his hand on the head of the animal so his guilt was transferred and the animal died in his place. Those sacrifices could only cover sin. They could not take it away. Jesus Christ was manifested to take away our sins, and in Him there is no sin. He died in your place upon the cross so that when you come to God confessing your guilt through Him, God can be both just and the forgiver.
So you should accept His mercy not because your willful sin was small, but because Christ is sufficient for willful transgression when you truly turn from it. That means you do not cover it, you do not excuse it, and you do not continue in it for pleasure. You confess it specifically to God, you agree with Him that this lust was evil in His sight, you ask Him to wash you thoroughly and cleanse you, and you forsake it. To forsake means to flee from it, to reckon your old self to have been crucified with Christ so that the body of sin no longer rules over you, and to turn away from the places and thoughts that feed that lust.
There are still consequences for sin even after forgiveness. David was forgiven and did not die, but his sin permeated his household like leaven through dough. God forgives the guilt, but He does not always remove the earthly effects, and that is part of why we are warned not to treat grace lightly. But the guilt itself, the heavy burden that makes you feel dried up inside and creates even a subconscious desire for punishment, can be removed only by confession and receiving cleansing from Him.
Do not let the enemy tell you that because you knew better and still chose pleasure, you are now outside of grace. That is the time to run to grace, not from it, but to run to it in repentance. Acknowledge your transgression without hiding, ask God to forgive you on the basis of what Jesus has done, and ask Him to create in you a clean heart and to teach you to flee from what once enticed you. The one who confesses and forsakes will find that God covers that sin with atonement and no longer counts it against him, and knows the blessed happiness of a load taken away.