D.B.
I'm going to leave you with 3 areas of thought:
1) Yes, give therapy more time. It takes a while for child and therapist to bond and for her to find out and experience the problems, let alone to implement strategies.
2) There is no rule that a 5 year old has to go to kindergarten. You can hold him out a year, find a pre-K or similar program, and give him an extra year to mature and get control. (We did this with our son, who had a June birthday. He was intellectually on track and socially adept in general, but he didn't have the focus he needed to be in a large class. And, he napped for 3 hours a day well past the age of 4 and I couldn't see him in afternoon kindergarten (which was the schedule for our neighborhood back then - half the district had morning K and half had afternoon K). He turned 6, and started K at 6 yrs. 2 months. Best decision we ever made. In the year in between, we put him in a Kindergarten readiness program 5 days a week with an extended day/lunch option 3 days a week, and he did great. He got used to the longer day, transitioned from the morning teacher/program to the afternoon one, and learned to adjust while still being in a smaller-than-public school class.
3) If you are "worried sick" and "feeling constantly defeated," you are making this a measure of your parenting abilities. It isn't. You can talk to the therapist about whether you are trying too many discipline techniques in too short a period of time, or not. But this is not about you, and it's likely that your stress and tension is adding to the problem. Not causing it, mind you, but not helping it either. Please work with the therapist or with another one for just yourself to help you learn calming techniques (which your son needs to learn too), and find ways out of this cycle of anxiety. If you get agitated, how does it help him to learn to calm himself down? And no matter how hard we think we are hiding our emotions from our kids, we aren't. You have other children and this stress level is not a good way to get thorough the next 15+ years of parenting. Yes, what you're going through is a trial and a frustration, but so is 3rd grade and so is 7th grade, and don't get me started on the teen years and the defiance/moodiness that goes along with it. Please get some help and some perspective for yourself. Asking for help doesn't mean you're a failure as a parent. It means your kids came without an operator's manual, and they are as different and challenging as a car vs. a vacuum vs. a lawnmower. Skills help.