Sorry I'm not much help hopefully someone else can help more.
All I can say is make sure you are completely comfortable with walk and trot. Just make yourself do it. That's what I do, I am still in fact building my confidence with certain horses cantering. One just because he is an extremely lazy ex-racer like you described. Then the other horse just goes as fast as he can and I fell off once which really knocked my confidence.
I rode the ex-racer on the weekend and found out his canter is the best apart keeping him going you don't have to work at all to sit to it, it is like a massive rocking chair.
Sorry for dribbling on but just remember to breath it helps a lot especially with the transitions and I find singing helps with getting rid of my nerves. Also if you can find someone, anyone who will push you. That is incredibly helpful my current instructer has boosted my confidence a lot because she knows when to push me.