I agree 100% with finding someone else to canter her regularly for you, until it becomes an unexciting event for your mare.
The second thing [I used to be terrified of cantering too, still am to some extent -my lesson horse while I was growing up had a severe bucking problem that would only appear at the canter-] that I've done to make myself feel better/safer is to ride in a pelham until I feel like I have the gait under control. Obviously that's a bit of a band-aid fix, but it's helped me.
I made sure to use two sets of reins when using a pelham and keep the curb rein pretty loose [unlike the even contact you typically want with a pelham]. That way I ride with the snaffle part of the pelham but have my "back-up brake" with the curb portion, if I need it.
My mare, luckily, is quite reliable undersaddle in the sense that she would never even think of bucking at the canter with a rider..but she used to appreciate a 'good' bolt every once in a while. The pelham helped me feel safe asking for a canter since I felt I could stop her and it DID help shut those bolts down, which kept me safe for real!
I've since been able to transition her out of the pelham. Now her "strong" bit is a simple ported snaffle [I actually had my farrier cut the shanks off the pelham I was using since she loved that bit but no longer needed the curb-action] and the gear we usually use is a rope halter with rein attachments. She respects it all now and I feel really safe on her back.
Of course, I would be quite unwilling to hop on a 'new' horse and canter off...but now I feel safe on Lacey and that's what counts!
Another thing that I found helped me feel comfortable riding the canter is to [we only canter out on the trail - my riding area in her pasture is not really ideal for much more than walking and short trots] have specific hills that we always canter up. I like that a hill gives me a specific start point - the bottom of the hill, and a specific end point - the top of the hill. Not to mention that I find an uphill canter to be much less intimidating than a canter on flat ground or downhill!
Since we always canter on the same hills [we do walk them upon occasion to keep everything sane but as long as Lacey isn't acting sassy about it, we usually run the same hills always] I just know that a canter is coming and it's less "Should we canter here? Well, here's a problem, and here's a problem, and here's a problem...cantering here would be too scary!". I could find reasons allllll day not to canter if I let myself.
That all being said, there really isn't anything wrong with just walking and trotting!