riding is much harder than people think. it takes time to develop the physical strength, and the balance. as you get more confident and have better balance, you can adapt to different horses, though it is always a challenge. I'll get on a horse, a different horse, like my frineds horse, and it will feel like riding a whole different species sometimes. I'll think, "how in the heck does this person deal with this rough gait?", and then I'll be glad to get back on "my" horse.
but, eventually , he get used to the new gait, and you no longer find it hard to deal with.
"leg on" all the time is what happens with school horses. they get so dull as a form of self defense that the require a lot of push to get them to go. in such a case, a crop is the better thing IMO, becuase you don't have to get all twisted up by straining to get leg on. you send the message "move!" with the crop, so you don't have to keep legging and legging.
as for the canter, chances are you are leaning forward as you strain to get the horse to break into the canter. this puts you off balance and the horse will trot faster to try and stay balanced under you (since you've pitched your weight forward), which makes it rough, which makes you curl into the fetal position and so on and so . you have to sit up straight, and really think of the canter as starting from the hind end, of you "scooping it up under you". this is something that you'll get in time, and for awhile it's going to be haphazard and a matter of luck much of the time. but, eventually, you'll get it. have faith.