The root cause of sin is self condemnation, I'm sure every time you indulge in porn the aftermath is always Guilt and most likely Stress. So you end up making new resolutions and promise yourself never to fall back to sin but eventually it's always the same cycle. Jesus Christ already paid the price for all your sins the moment he died on the cross so rest in his finished work and start experiencing victory in your life. The devil always condemns to make us feel worthless in the presence of God but that's all a lie, God will never forsake us. I want you to always confess this statement 'I AM THE RIGHTEOUSNESS OF GOD IN CHRIST JESUS' every time you get temptations and after sin and i grantee you to be set free from the bondage of lust. The reason you can't overcome sin is because of self-righteousness which makes us rely on ourselves for salvation. You got to be Christ conscious in your life to reign over sin so i can promise you one thing there's light at the end of tunnel.
Be still and strong, The Lord's is in control
Please read this book:
https://www.academia.edu/29987855/Free_.pdf_download_DESTINED_TO_REIGN_-_Joseph_Prince_Ebook_.pdf