You are over thinking it - you shouldn't have to be in a mind set to canter it should really be a part of your riding, and sure bad things have happened to you at the canter. You have to forget about them. When you start saying "Ok time to canter... now, ok maybe now, ok maybe now, no wait too worked up, lets do it later." stop yourself, breathe and say to yourself "We are cantering NOW". Make it definite - don't make excuses to yourself while you are riding just do it. When you have a nice, lovely trot breathe out, picture having a nice quiet canter, and ask for it! If you start worrying about it the horse can tell and then in turn they worry which makes you worry more - so stop worrying!

Another idea would be to grab a lunge lesson if you could.