Even though you want something with draft in it, i think quarter horses are the way to go, especially if your nervous. most quarter horses, that arent sprint bred, are quite thick, and have alot of bone. They only reach around 15hh, so this may be an alternative, as they arent so big! (: i have three quarter horses, and their temperament is great. they are quiet, and easy to anything with, but can get up and go when needed.
Hopefully you find something!! (: