Cutting In

I have been having some problems with my horse cutting in when I ask him to transition to a faster gait. My instructor and I have been working on this for a while now, and it doesn't seem to be getting any better. I'm starting to get a little frustrated.
Thanks in advance.

Try to fix it before it happens. Meaning, over bend him a little a push him out hard before asking him to go to the next gait (both downward and upward). With the info you gave this is all I can tell you. But I hope it helps! Good luck!