Hmm, idk, that almost feels too limiting. Like you wouldn't want to use it lest you turn into a monster/lose yourself, etc. Maybe it's more like magic requires a lot of energy and that energy is easiest to pull from yourself (and its taxing, but not so taxing it's impossible to use) but you can pull from other things too, like nearby animals or plants. And  you have to be careful when doing that or you can obliterate plants and kill creatures you didn't mean to. Could also be that since this magic is kind of a living source, that maybe when you tap into it, if you're not careful, you can get overwhelmed in a physical and mental sense and sort of lose yourself (temporarily) if you reach in too deep.