Mana is toxic
Mana is like radiation, it progresivelly acumulateprogressively accumulates in our body and we aren't able to get rid of its damages. I mean, you can spend mana on a spell, but the damages that it has provoked in your body are'taren't healable.
And if you try to use mana on spells it won't reduce your damages, it'll do the opposite. Think about bleach, I have read one time that if you accidentally drank bleach you have not to vomit it because it will damage twice your esophagus and throat (damage when in, and damage when out). If you spent mana, it'll burn inside you twice.
Mana is a subproduct of life
Each second we live, we breathbreathe or move, a particle of life force is burned from our soul in order to produce that "energy" we need to breathbreathe, move o even think. But like the second law of thermodinamicthermodynamics say, any process can have a 100% of efficiency, even flames produces heat and light.
That is your mana. Your life is coal, when you burn coal it produces the light of your everyday life (which you spent to move, talk, play, work, etc) but also it produces heat an ashes which actually theirthey are your mana.
Basically, that heat representrepresents your mana energy used to cast a spell (when more you burn more warmer you are), while your ashes may (if you want) represent the acumulatedaccumulated damage of your old body.