And that is what frightens her.
Greater power leads to greater risk of corruption, and Willow is taking the magic far too lightly. That's why Tara knows she must leave - she loves Willow, but she thinks perhaps the shock will wake Willow up.
Tara's power is a candle, but Willow's is the fire of a thousand suns. And Tara worries that Willow will burn up in it.