Glitching Iphone Screen

My iphone 8 has been glitching and it continues to get worse. It started off at the bottom of my phone and now it has worked it’s way all the way to the top. However, if my brightness is turned all the way down, the glitching stops, but if the brightness is turned up, it starts glitching again. It doesn’t affect anything with the phone, like the touch screen, but I can’t see a thing when it’s glitching, and I cant keep my brightness down forever. How do fix this and how much will it cost?