I was quite a successful teacher for several years before having my child. They even specifically put "tough" kids in my class for me to work because they knew I could handle them and I still have similar challenges with my child with SPD. You know back in the day when they were first learning about Autism and they had the theory that the "refrigerator mother" theory that children with Autism were that way because they mothers were cold and uncaring? I think the "mother needs to be firmer/child is undisciplined" is the SPD version of this. It's really not your fault.

I really like the book Sensational Kids if you are wanting to read more. I've also found a lot of help in biomedical treatments with diet and supplements, borrowing from what those who treat kids with ADHD and Autism have learned

Healing the New Childhood Epidemics
gives a lot of information on this. Dr. Sear's Autism book
gives a brief mention to SPD and is a good outline of diet and supplement treatments as well as many other treatments.