Pastrygirl, I feel like I am always chanting this on this forum but SELF-CARE!!! Of course you can't reward every 10 minutes, etc, these are super-human expectations. Therapists can do it because they are trained professionals working for a set number of hours--not around the clock with laundry and cooking and everything else that goes on in a busy family. I would seriously try to build in more de-stressing time: therapy, massage, taking a walk, coffee with a friend. I know it is hard and sometimes expensive to make that investment but you really do need to take care of you. You sound a little burnt out to me, understandably so.
Just to throw this out there. I wonder if a big part of your reaction is anxiety. It is anxiety-provoking in the extreme to see your child acting very differently and to see their (sorry I don't have a better word here) deficits. It is scary and painful to watch. And I think those emotions get sort of mixed up in or projected onto feelings of anger and frustration with the child. Certainly, I acknowledge you are feeling some legitimate anger and frustration as this is a hard situation! But the "super-charged" nature of how it feels to you I suspect may be due to the absolutely expected, logical, and understandable anxiety and upset provoked by watching your child's differences--which are all the more evident to you I think once you mentally switch into a public context like the karate class. Hope you don't mind my arm chair theorizing. I just don't want you to feel worse. You clearly love and are devoted to your DS --look at all you are describing doing with him despite the difficulties. I suspect it is the anxiety and upset from watching him that is sort of getting projected onto him, not that you actually care so much about his karate moves, kwim.