As someone with RSD, I was involved in multiple activities - some professionally, some not - that involve receiving, processing, and using critique long before I started writing fic. And what it comes down to for me is really giving myself time for that "processing" bit.
Whenever I'm receiving critique - whether it's on a fic, a performance, an academic proposal, whatever - it's a multi-step process for me.
1) I make sure I am ready to face it. In the case of beta comments or a submission or something, I see that it has come in and then put it aside and wait until I'm in a good headspace before reading it. In other cases, the critique may be live as I'm doing the thing, in which case I make sure I'm in that headspace before going to do the thing. (The rest I'm only going to talk about critiques I can deal with on my timeline in writing bc realtime is so different)
2) I open and read it. Read it through once, put it away, and let myself have whatever emotional reaction I'm going to have to it. If I need to cry, or curl up in a ball for a little while, or whatever, I do that. I generally don't let myself proceed beyond this for at LEAST a day, as sleeping often resets a lot of this for me. Under no circumstances should I try to engage with it again until I'm no longer actively spiraling about it - when I say I read it ONCE and then put it away, I mean it, no staring at it obsessing.
If you have trouble ever pulling out of that spiral, like ao3cotd said, talk to a doctor, that might be a therapy thing. Generally with a night's sleep worth of distance I can see how my initial emotional response was an RSD overreaction and that makes it easier to put the actual critique in perspective.
3) Once I'm feeling okay again, time to go actually deal with it. First is reading it again with that added distance and knowing what to expect, then considering each piece of criticism individually.
4) For each one, the first question is: do I agree? You don't actually have to take every piece of advice a beta gives! I recommend considering each one carefully and not dismissing anything out of hand no matter how tempting, but sometimes I spend quite a bit of time with it and in the end go no, I am actually okay with how I did this. Or possibly, yes this is not quite as good as it could be but fixing it would be more time/effort than it is personally worth to me.
4b) (optional) Talk to the beta about anything you're unsure about or don't understand where they're coming from. This can also involve explaining/defending a choice you made but a) generally I wouldn't, just don't take the advice if you don't want to and b) if you do, do it from less of a place of defense and more of a place of "ok I see what you mean but here is why I did that and I don't think what you said accomplishes that, how do you think I can address what you see without losing what I like about it?"
5) Finally, start actually implementing the advice, which often also involves talking to the beta as you go ("if I do this instead of that do you think that would fix it," "what do you think of this restructured scene," etc). If needed, take the time in steps 1-3 when approaching this additional feedback.
I think this is all really important to learn how to do when you have RSD! I had no idea that was a thing (or that I had adhd) when I started having to learn how to deal with my overreactions to critique, and learning about it made me feel a lot less silly about having to use this process.
(This is also why I do not appreciate critique in fic comments, because I have no way of knowing it's there before opening the comment and BOY can that destroy the rest of my day.)
