Sorry for the really delayed reply, haven't really been using the forum much as of late. But I wanted to reply anyway.
I've had ups and downs over the past few months with regards to my feelings about blushing, but I can say with some confidence that I have a more positive attitude towards it now. I hope this inspires you with some confidence.
I don't know if you've ever tried talking to someone you trust about it, but for me this has really helped. Of course, some people notice blushing and call you out on it (which is horrible), but I had a really long conversation with my boyfriend quite recently about it and he told me that he was very surprised to learn that i had this phobia - which to me was implausible, and I genuinely didn't believe it at first. But as we talked it out, he really made me believe that unlike the likes of me and you, most people aren't looking for blushing, so most of the time don't notice it (and trust me, I really do go red, so I was just as surprised as you!)
He went onto say that he's sure he's witnessed many people blush in his life, but that he just doesn't register it because it's NOT A BIG DEAL to him. He even said that if he did notice someone blush, he may register it for a second but move on, or even put it down to someone being warm. Either way, people do not judge us the way we think we are being judged in our heads. 99% of the population do not even know this is a phobia, so will not view us as crazily shy like we think we are being judged. Even the people that called you out on it in the playground noticed your blushing, but probably didn't fathom the impact it would have on you - because to most people, it's simply not a big deal. It's a hard concept to get your head around at first, but that's because our beliefs have been there for so long that we see them as the ultimate truth. But I encourage you to think about this for a minute, and try to understand how it might be possible that to other people, blushing isn't a big deal, and therefore aren't judging us for it. People have phobias of all sorts of things: snakes, water, vomiting, things with wholes in. Do you think that all of these things are the be and end all? No. Just like how someone sees you blush and they don't care (EVEN if they mention it - they will have forgotten about it in 5 seconds, unlike us).
Easier said than done, but it's also so important to remember that blushing does not determine our self worth. Really, it's just a physiological reaction, so when you think about it rationally, it's silly to let it determine how we feel about ourselves - it's like feeling worthless because we get headaches. Meditation has really helped me to get in touch with who I am on a larger scale (i.e not just a blushing mess) and all of my good qualities. This really helps me to rationalise my thoughts when they crop up. People - my boyrfriend included - see us for a lot more than just our blushing (if blushing even registers for them at all in our characteristics!) so I encourage you to ask someone you know well to describe you, and see if they use 'blusher' as an adjective. I guarantee you they won't.
Sorry, this was kind of long, but I hope it helped. Be proud of yourself for even taking the time to combat SA, and congratulate yourself for all the hard days you've made it through already. You may not acknowledge it, but you're a lot stronger than you think for even having these feats. Good luck!