It hurts extremely because your whole body is physically altered.

You shift by magically turning yourself inside out ass first

Your brain shape shifts as well into an exact copy of the animal you mimic. You have the intelligence of the creature you turn into. Which means a one way trip as any coherent logical human thoughts is driven out by animal instincts.

Your clothes are not transformed with you

Your weight also change, but the difference is converted to energy using mass–energy equivalence formula.

But comes with body dysmorphic disorder. You’ll never be satisfied with how you look.

Granted, but you can never turn back

You can only shapeshift into smaller versions of yourself.

You can only shift into a species that you've seen before. Can only shape shift into living things (not sentient and mobile inanimate objects)

And you can never return to any species you were previously. It has to be a new one every time until you exhaust the possible species. Leaving you stuck as the last possible species you pick to shape shift into.

you shapeshift, but your state and society will periodically and randomly remind you, that they know, what shape you had earlier ... wait, that does already exist. still superpower tho!

You can't actually become anything unique, the body approximately closest to your imagination becomes yours. They also, become you in turn. You immediately become a world wide sensation if used frequently.

Your telomeres become damaged every time you use the ability

You forget what you like if you are changed for too long and don't have a detailed enough reference

You can also become invisible