Well the way I thought up Eilis is basically that she is a conductor, like metal for example, normally when she was unbound (the tattoo's) she wouldn't have problems calling forth and releasing magic. As much as metal can conduct heat. However too much heat and you know what happens.
Okay, forget everything I said. That's a much better analogy than what I had, we'll go with that. To bounce off that idea we could also say that every time a mage uses magic, they get a little bit of feedback in the form of, say, waste heat or something. Too much and it burns them out from the inside like what would happen at birth if they couldn't achieve balance between their own soul and their connection with Gaia.
Or instead of just heat, we could also say that a little bit of what they cast goes back to them. Eg. slight recoil from telekinesis, a cold feeling from cryokinesis, a mild buzz from electrokinesis, that sort of thing and it gets worse and worse the closer they get to their limit. This feedback can be lessened through training, but never entirely mitigated so it's not a matter of stamina and focus, or running out of mana but more rather gradually losing the ability to do it safely like an overworked muscle.
So it's like you can have as much coffee, sugar and stimulants as you want, be all wide awake with energy to burn, but if that muscle hasn't had time to recover, it
will tear and it will hurt. Except for mages, it could go from 0 to 100 real quick because of the forces they're working with. They'll experience pain at first, an early warning sign of reaching their limits, then once they get past the point of no return,
fwoosh. You end up setting yourself on fire instead of the other guy. Or get frozen into a popsicle. Or get blown away by your own telekinetic push, etc.
How's all that sound? Oh and I'll have to put off posting til tomorrow again, I kinda got a bit carried away with all this.