It takes time to toughen up, it took about
six weeks for me. Warm compresses will likely help, spend as much time as possible without a shirt on or anything rubbing up against the breasts. After you're done nursing you should express a little more milk, rub over both nipples and areolas, and let it air dry.
Talk to a lactation consultant. They can help you out a lot to make sure your latch is good and that you're not having any other issues.
Don't give up. You're doing great.