H.K.
It sounds like you could benefit from a parenting class. I know with your "paraprofessional education" you might think you know everything to be done with kids but a good parenting class can teach you some tricks to help gain your sons compliance.
You may also have to face the fact that you may have a troubled little boy. I am the mother of a young man diagnosed as bipolar at the age of 7 (spent many weeks in mental hospitals during his childhood). His problems really started to become evident about the same age as your son.
I used to carry around 3x5 cards that said "Thank you for your concern. My child suffers from a severe mental illness that exhibits itself in the behaviour you are whitnessing. While your input is interesting, it is of no use to me at this moment and I will leave his care to the professionals that we are involved with. If you have further interest in childhood mental illness please visit the National Alliance of Mentally Ill (NAMI) website." I used to pass those out to the nasty people who suggested that my son would benefit from a spanking when he misbehaved in public situations.
It can be a struggle to raise a willful child...I presume you are doing this alone since you made no mention of a husband. I would get your son a proper psychiatric evalation and get yourself enrolled in a parenting class. Once you know what the problem is..."bad" parenting or something more serious you can fix it and then people won't judge you so much.
You do have to understand it is a perfectly reasonable assumption to assume you are "not a great mom" when your child acts up. Although it may not be the case at all. I wish you well in your search for answers.