I'd have to disagree with the lazy Warmblood statement ;] All of the ones I've ridden have been feisty.
Rearing is a very dangerous habit, and so is taking off when other horses run by. You'll have to, have to, have to accustom her to being around other running horses. Maybe lunge and ride her in a paddock with 2-3 horses, and make her listen to you. If you feel her tensing up as other horses pass, back her up or turn her in a circle to get her focused. Rearing is scary and VERY dangerous!
Congrats on your new horsey, hope things work out! :]
I'm so busy, I don't know if I've found a rope... or lost my horse...