Stopping milk supply

My baby is 4.5 weeks old and mainly formula fed. I've been expressing to be can have my milk and tried to get him on the breast now and again but I always end up in pain. Since Thursday my nipples have been extremely painful and since yesterday my breasts in generally really painful. So bad I can barely put my baby on my chest.

I just want to give up now and get rid of my milk supply. I've tried as hard as I can but I just can't do it anymore. I want to be pain free and happy.

How do I stop the supply?