How do I stop my nipple from chafing?

The problem isn’t the nipple, it’s the shirt.

Don’t wear that shirt!

A possible temporary remedy would be to put some tape inside the shirt to get a smooth surface. I’m thinking clear packing tape might work. If that won’t stick well enough, a package of “Hello, I’m ___” labels or something similar should do it–they’re meant to adhere to fabric.