If you do use Mythic Earth, there are two revisions I suggest you make for balance sake. These did not come up in my game until after I'd published the book.
First, any spell with a duration of 10 minutes or longer should incur a -1 penalty to spellcasting checks, as should things like teleportation and divination that drastically shorten the amount of time it normally takes to accomplish something. Moving 100 miles by car takes a while, but teleporting does it instantaneously. Researching takes a long time, but divination lets you just think of the question and get the answer. So for any spell with a duration of 10 minutes or longer, or for any spell above 4th level that involves teleporting, divining, or knowing the future, have it incur a -1 penalty to spellcasting checks for the rest of the day, regardless of whether the spell succeeds.
Second, I made Cure way too expensive. It should cost 3 levels to heal 1d6 damage. However, I recommend Cure also incur a -1 spellcasting penalty, to keep it in check.
For psionics, you could use Mythic Earth to thematically represent the idea that your willpower and concentration directs your power, and as you exert yourself you can always keep trying, but eventually the strain on your mind becomes too great.
