What should I do when I commit a sin and feel guilty and shameful?

Whenever you commit a sin, do this. Give some sadaqah. The Prophet sallallahu alayhi wa sallam said, As-sadaqatu sadaqah, it extinguishes sins as water puts out fire, meaning it completely wipes it away. And make it something upon yourself that every time you commit that sin, you give 10 pounds or 20 pounds. Have a fixed amount that every time you commit the sin, you do that, and that will really take your soul, your nafs, away from that sin. Number two, stay away and cut off the avenues that lead you to that sin. Maybe it's going to a particular place, you know, replace those with other things that will take you towards Jannah. And inshallah, you will see that your heart will be purer.