For pasture board, do you get to supply your own feed for her? Is it a dry lot or is there actual grazing for her? If so, I'd say just pasture board her for a few months and see how she does. You might even find that she'll gain weight even when getting less food because her metabolism will regulate itself better with constant access to exercise AND food. I know that our TBs do tremendousely better when they can move around whever they want to. They also keep their condition better and are much easier to handle.
You might also look at trying to find another boarding stable that can accomodate her better. One with pasture board that has maybe 2 or three other pasture mates and you can bring your own feed/she gets fed seperately. There really is no quick fix for this and letting her canter from the start when you ride her is a very bad idea because you won't get anywhere with your training. Actually, IMO, you shouldnt eve be letting her canter AT ALL yet if you've only had her for a month. We gave our OTTBs ATLEAST a 3 month let down from the track where we did only ground work and let them be horses in the pasture, and I didnt start cantering them until the 5th or 6th month of owning them...and only if they walked and trotted quietly first. Besides that you wil be teaching her that running when she wants to is ok and that she can get away with crazy behavior. Thats the last thing that you need to have in her head.
Whether it be finding someone to lunge her or let her play in the arena every day, finding a place where she can be turned out at least half of the day, or better yet- 24/7 pasture boarded- you do need to deal with the situation, not just her behavior under saddle. ANY horse will go stir crazy when living in a stall and only coming out to be ridden because it just isnt natural for them, but a young race-fit, race-fed OTTB will be even worse. You need to think about her mental wellbeing, not just how to get her to behave.
our 'Crazy' OTTB gelding, two and a half years off of the track. 24/7 pasture with 8 other horses, only getting rice bran, free choice hay, and corn oil, low man on the totem pole (dispite being 17.3hh) and as you can see- happy, healthy, and absolutely too high energy for us to deal with ;) in fact I think he's about to eat that little mini and bolt on that little girl!