How do you get a horse to collect its canter

She loves to canter, but she tears around the ring.

When I first got my horse, she had no idea how to balance herself under saddle, so trotting and especially cantering was our big issue - she would bolt when you asked her to canter and pick up the wrong lead to boot - it was a bit dangerous since she would just go like a downhill freight train - which she basically was. This is what we did:

Basically, my instructor made me ride 20m circles constantly - I don't think I was allowed to ride my horse outside of that circle for at least half a year (the occasionally diagonal, but I wasn't allowed to go down the length of the arena because my horse would bolt and loose balance). It was a lot of half halts, sitting back with my shoulders back, head up and deep in the saddle. For a horse to collect it's canter, funny enough, it takes a lot of leg. You don't want to collect, just ride a normal canter. The thing is, it's a lot less effort for the horse to run around like a psycho - yes, it's exhausting, but she's not using her body and muscle to stay balanced. When you give half-halts, you need to make sure that you are at the same time giving her a lot of leg. You don't want her behind your leg, you want her in front of your leg, giving impulsion from the hindquarters and through the back. Also, half-halts, the horse will say "oh, slow down" and go into a brisk trot because, again, its less effort than balancing themselves, so your leg needs to keep her cantering while the half halts bring her back into your hand. You need to have a soft, but firm hand - I had trouble with this, so my instructor had me constantly moving my fingers as if I was playing the piano and basically my thumb held the reins by pressing into my top finger. If your hand isn't soft, the horse will never come into it because it hurts their mouth and you'll develop a tough-mouthed horse which will need a harsher bit. You need to be firm though with the half halts to say "hey, cut it out."