Best way to get this pony to canter?

Sounds like you love this pony very much. From what it sounds like and from my previous experience with similar horses you need to find what gets her the most forward going. If she is more forwards going on a hack keep askin her on a hack, if she likes jumping try getting her to canter towards the jump. This generally makes the horse understand the asking for forwardsness better as they already want to go forwards. You will have to keep practicing and practicing and that fact that she is a pony will make it harder because they have a fast trot naturally and so when you push them on they rush even more and it becomes harder to canter. What you should do is either on a straight while hacking or going towards a jump ask her and keep pushing until she breaks into it or while your on the last half of a circle just sit down and give her the canter cue, and keep pushing her forwards. When she does eventually break into the canter, allow a couple of strides, pat her then ask her to come back to trot. Even though she isnt a youngster from whaqt you have said she hasnt had much done with her riding wise but you will have to treat her in a simarly way. Hope this helps and good luck with her x