My son has the perfection issue going on, slightly less intense than you describe your son. He never had the separation anxiety at all though, but in all the other things you describe he sounds just like your son - the happy, outgoing, friendly, well liked, physically very capable. His emotional lability didn't really appear until around when he turned 5. Up to that point, he just had what appeared to be the upper range of (age appropriate) low frustration tolerance. Last year he held it together all year in Kindy except for one time in the last week of school when he misheard a direction given and colored the wrong thing on a worksheet - he kind of lost it in class for a few minutes. Other than that one time though, he'd save his breakdowns for home. I would always know when he had a stressful day at school, because he would start crying over something seemingly insignificant and would go on for an hour about it, then finally admit he was frustrated about something at school. Any time he "blames" his tears on something that normally doesn't bother him, we know there's something bigger related to the perfection issue going on that happened in the past couple days.
It breaks my heart that he has these perfectionist tendencies, as we've never been the kind of parents that tell him he's doing something wrong, or not well, or whatever. He's been in play therapy for a true phobia he needed help getting through and to help work through the perfectionism....the phobia is gone, but the perfectionism lingers. His therapist says that he's very emotionally perceptive, and pretty advanced in emotional IQ...however, his social/intellectual maturity is still at age 6 so it's a lot of turmoil. He gets choked up when I sing "You are my sunshine" to him, he teared up at the end of Toy Story just like I did; he seems to "get" deeper, more advanced emotional things and articulates pretty deep emotional things. Which again, is great when you're able to handle them and cope. Not so great when you're 6.
Many people think perfectionism is a positive thing to have, but it's really not. It's feeling like you're worthless if you don't do things the way you wanted to, feeling like the day is ruined if things don't go the way you pictured they would, and thinking no matter what you do it's not good enough - what could possibly be good about that? Wanting to achieve and expecting your personal best is positive, expecting perfection is NOT.
One thing that helps with the breakdowns is staying away from dairy. We found that if he has dairy more than a couple times a week (and I mean major dairy sources like milk, cheese, yogurt, ice cream) he breaks down easily over things, butt if we keep him down to a serving or two a week he's much more able to handle things without crying and mentally beating himself up. He beats himself up for whatever it is he did "wrong", and then beats himself up for being so upset about it, so it's a double whammy.
I don't have much else to offer, other than comisseration and the idea about restricting dairy if he has it regularly. It took about 2 weeks until we saw a difference, and if we slack off and let him have more for a week, he starts melting down and we can see it within a few days. I wish I knew how to reach him to let him know that perfection is unrealistic, and that just because he does things he shouldn't do or doesn't do everything perfectly the first time every time, he's not a bad kid or dumb. It's so heartbreaking to hear your 6 yo say that about himself, and in that moment really, really mean it.
He absolutely refuses to do any of the work I've seen on processing perfectionism, making mistakes on purpose, boosting self confidence, etc. He just flatly refuses. We got a book, "When good enough isn't good enough", but he won't read it with me. We bought guided meditation CDs, but he won't listen to them. He's convinced that this is the way he's meant to be and that nothing can change him. My glimmer of hope is that it's not *always*, and that after he takes his own tiem to process things, he seems to be able to get over them, it just takes a LONG time, and a LOT of tears and negative self speak. Sometimes, he can handle imperfection. Sometimes, he can even laugh about it....but other times, he so, so hard on himself. Last night he cried for an hour because he mistakenly thought his sister was goofing off but had actually hurt herself, and he laughed at her before he realized she was actually hurt.
He was upset through the night and still this morning, and it took him about 2 hours this morning before he piped up out of the blue, "mom, I feel better about what happened last night." So there IS hope.
Editing again to add that things that might not *seem* like they are perfection issues can actually be perfection issues...like even the friend not sharing the slide; in his mind, friends share. Period. So if the friend is not sharing either he did somehting wrong and the friend is mad at him, or maybe they're not really friends, or maybe he didn't describe the game right, or...or.....or.....so it's more than just the friend not sharing the slide.
My son's world is a lot of absolutes. Either good or bad. Either smart or dumb. And if you do the negative thing even one time, the other 99 times you do the positive thing don't counteract that, you ARE the negative thing (in the moment of frustration).
Actually, typing this out has really, really helped me get a little more perspective and better handle on this - so thank you! I'm so glad I clicked this thread!!