I wish you the very best with your new horses. Do not carry guilt about something done by other persons – there was nothing you could have done about it, short of keeping your horse with you at all times, which is impossible.
One of my horses is an off track Arabian. At 24, he still thinks life is a race and he should win. Thoroughbreds are much quieter. One of the things I like about former racehorses is that they are so accustomed to people asking them to do odd things that they tend to be much easier to handle than the average horse.
Horses always pretend they are starving. Nature intended them to just eat grasses, and graze all the time, and we (all people) have interfered with that. If a horse is not working, it really doesn’t need grain at all barring some health issue (such as older horses who cannot maintain their weight).
Post updates on your progress – there is nothing better that hearing how people progress with their horses and their accomplishments.
It is never the horse's fault