I figured it would be from being unbalanced but he does it after 2 strides of canter consistently every time, and once I get him past that he's great, has a nice relaxed canter.
And we are riding with different saddles and bridles, but he responds well to both of them, the only difference is my saddle is about 7 pounds heavier, but both are western. Thanks for your help!
Do you know how well he's been trained? Does she have any lessons (your girlfriend) under her belt? If I'm prodding too much, let me know.
My horse can do a lot of strides by himself at the canter with me on his back, but he often gets a little un-balanced so we trot and pick it up again.
He could be falling into the canter, which is why he consistently gets 2 strides in then falls apart. Try to get more put together before you ask for more. I could go into detail but I'm still grasping all of it myself. Though lateral work like leg yields on and off the rail, circles, serpentines, etc. will help. Also maybe instead of pushing him on, keep transitioning between trot and canter.. like 10 strides canter, back to trot for 5, canter again for 15, back to trot for 5. That way the horse will be paying more attention to you as well.
Do you believe in lunging/is the horse trained to lunge? It helps to work them at the lunge through those transitions or to get them cantering consistently. It's not for everyone, but it sure has helped my green horse who canters much better now that he's getting lunged consistently beforehand.
And I have no experience with curb bits so I can't help there, sorry :/