It doesn't hurt physically.

Applejack can't feel the weight on her back when carrying the apples or the sore in her legs after working all day. Her body had grown accustomed to the heat in the fields and the splinters inside her hooves.  It never hurts when she is moving and doing her chores.

When it hurts is at night, when Applejack can't sleep.

It hurts when looking at Big McIntosh´s empty room, abandoned after he married and made a family of his own.

It hurts when visiting the hill where Granny Smith rests along with the other family members that would never leave the farm.

It even hurts when it shouldn't, when looking at Apple Bloom working in her workshop. She should be proud of her, so talented and dedicated to her work but it still hurts deep inside.

The pain doesn't stop at the orchards, is still there when visiting her friends. When watching Rainbow do a performance, when talking with Twilight at the Palace. When noticing how much they have grown and how far they have gone.

It hurts because Applejack hasn't changed while the others continued with her lives. She is still the same, always working, always supporting her friends and family. Never demanding something for herself, never indulging on any selfish desire. She keeps on giving, on listening, on offering a shoulder to cry on or a proud smile from the crowd.

She is still there in the background, giving it all for them. No matter how much it hurts at night, when she is all alone.

She ignores the pain at her chest and goes to sleep.

She has work to do the next morning.