For a decent sitting trot or canter, you need to move your hips with the horse. The trot can be easier because there isn't as much power flowing thru the horse's back, but it can be tougher because the pulses come faster. This video is from a western perspective on cantering, but I think it does a good job of showing how your hips need to move for either a sitting trot or canter.
If fear is making you tense, then some options would be a lunge line, or maybe trying it with either a western or Australian saddle. If I'm willing to 'cheat' sometimes to help my mare past her fears, then why not 'cheat' myself and use tack that can help me get over the hurdle. The feel of a sitting trot or canter is very similar regardless of saddle type, IMHO.