Some of you might know of a post I made, talking about the physics of a bender's body, as well as what would happen if their bodies were overloaded. Here's the new, revised logic. Enjoy.
Benders channel the power of the lion turtles through their bodies. Obviously, (due to the amount of power they use up while bending) this power cannot come from their own bodies. All the bender does is call on the energy and aim. So much energy coursing through their body obviously makes them fatigued, but not so much as it would should the energy have come from their own body.
To channel the energy, which we'll just call "ether", the bender uses an organ which we'll call the jing (I'll get to why I called it that in a second). When they call on the ether but don't release it, the jing is overloaded.
The jing will become inflamed and cause the bender immense pain, similar to having an inflamed appendix. If the bender survives, they will be unable to bend again in their life & the jing will never recover. However, they will be able to call on the power. If they do this with their jing damaged, the jing will pop inside them, and the energy will escape. It will take a form at random, commonly heat, kinetic or radioactive energy.
The bender's death will look like the Ender Dragon death in Minecraft, but faster and with a lot more gore.
Another thing about the jing: in AtLA, Bumi mentions positive jing, neutral jing and negative jing. Positive jing involves attacking, negative involves retreating, defending or evading and neutral involves waiting or "doing nothing".
The firebenders specialize in positive jing, the airbenders specialize in negative jing, the earthbenders specialize in neutral and the waterbenders have a mix of all three.
The jing (organ) is directly linked to jing (concept). Use of power is good for the jing (organ) but too much of the same sort in unhealthy. This does not affect the actual bending ability, but can cause problems elsewhere, mostly in the brain.
This is why Azula, Kuvira and various others became mentally unwell, not only due to their mindset, but also due to an unhealthy jing, because they only used positive jing.
There may be other medical setbacks of an unhealthy jing, and I'd enjoy to hear suggestions.
Anyway, enjoy your nightmares.