One of the best books on dealing with challenging kids I’ve ever read was Dr. Ross Green’s The Explosive Child. It changed so much for us. Dr. Green has a radio talk show on Tuesdays and takes questions on how to help your easily frustrated child do well. The shows are archived so you can take it in at your convenience.
If you are moved to do so, give it a listen.
If your child has flexibility issues, I highly recommend reading the book. And remember, as Dr. Green says, (paraphrased) when you are dealing with an inflexible person, it is asking for disaster to match their inflexibility with your own.