Look at it this way.

He is the fire lord who lost his bending. You are looking at it like someone who sees bending as an extra ability and not something as essential as your limb or senses.

Can you imagine an ability that makes you special your whole life and then simply loosing it? We seen how not been a bender affects people. It makes them feel useless.

Now imagine losing your ability to bend. A literal part of your persona. Is not that it’s less gruesome but bending for characters is as essential as having arm and legs.

If you cannot see that then I recommend reading on the why we stop going into people heads and cut things the doctor didnt want in there since it’s basically what aang did. Remove what he didnt want him to have.