In me you can see your reflection
This line makes me think of a mirror or water - they both have reflective qualities.

A life sustaining substance keeps me company
This makes me think of water even more, but it could also be mist. What does it keep the company of?

I am taller than most others
It's the tallest thing about. I'm wondering if it's the Clyra tree, but that does seem like the answer....I'm not certain if the answer if a living thing (monsters, sentient life being (Iifa Tree boss; Necroron[sp?]); or people (The Qu Clan; the Mages; the Summoners; etc.))

I have been broken
This makes me think it's a mirror, it could also be a statue. It could be a metaphor for a broken heart, or something else.

What am I?
I don't rightly know, I'm going to ask if it's related to Terra - as in, did it originate from Terra and, consequently, Garland?