There's a difference b/t normal teenage angst & someone who never learned properly when they were younger that they aren't the centre of the universe (which eventually leads to healthy self-reliance). There are a lot more young ppl like that nowadays (mostly due to ineffective, overcompensating parents) and by the time they reach teenage years it usually requires professional intervention.
Be supportive in a friendly way ('yeah, I hear ya' type thing), but if they are more demanding or needy than that, this will be beyond your ability to deal with. Encourage her to seek help. Counselling services at your school will know what to do.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh