1. She was more of a pasture pet before I got her, and was just used as a trail horse when she was ridden &
2. She is overweight and I think that she definitely needs to build up some muscle first before she has enough balance for a nice slow paced canter.
My trainer rode her the other day and tried to ask for it...she basically did the same thing, ran into it. He got her to slow a little bit but then she just broke into a trot. So I think we'll just need to keep her working at the walk/trot awhile first to build up her balance again. We've been working on her circles in the ring to help with that, and I think she'll get it eventually. He mentioned to me that I should just lounge her every now and then (instead of riding her that day), since she'll canter on a lounge line for me. Then she will be getting some of her balance back that way too.
For now, I'm not going to push her, because as a couple of you touched on above, I don't want her to learn any bad habits by just running into it and/or me having to make her stop. I agree with both Sonny and Appylover above in that it's probably not a good idea to let her run into it because then she'll want to do it all the time instead of a nice transition, but that I don't want to allow her to stop altogether either and learn that she can just get out of anything I ask her for by stopping. I think you both have some very valid points there!
Thanks everyone, I'll keep you posted on her progress!!