Here is something I can speak to directly.
I too used to hop in the car and go somewhere else for a while, just to chill out myself. This was before I even knew what a meltdown was. I just knew something I hadn't seen before had happened, and it rattled me, and I couldn't derail it or stop it happening so I just had to leave a couple hours or so.
As time went by I really wanted to understand what was happening. Leaving out a lot of personal stuff from a public forum
, eventually I read and figured out what was going on.
It was a huge relief to at least know it wasn't me and that there truly was nothing I could do about it, once it was happening. That allowed me to disconnect emotionally and not blame myself and feel extra crappy on top of how crappy and powerless I already felt seeing my loved one suffer through said meltdown.
I also began to notice that he always seemed to be OK afterward and frequently did not even remember anything leading up to it or about it or after it. :/
Sometimes it still affects me to the point I can't sleep afterward. Mainly that is if I feel like there was a communication problem and I am somewhat to blame. But other times I haven't even seen him that day. I have learned it's something he needs to do at times to process excessive buildup of stimuli, or something like that?
So if you have a problem of feeling bad and guilty and blaming yourself, maybe some of that will help you.
One thing you can do is just with time learn to notice what he is like before and after. Maybe you can even see signs a meltdown is building up and warn him so he can go chill or isolate or whatever, because sometimes that seems to stop it happening. I pretty much 'let' him do what he wants to because I'm fairly introverted anyway and I need my own time alone and he does even more so. I don't take that personally. It is how he is wired. So, less social pressure, even with you at times, could help.
Also, discover the joys of single hobbies, headphones, itunes, computer games...anything you can do alone that absorbs your focus. I fight stress a lot because of all of this, but, those are crucial coping mechanisms. If you have a support system, reach out for it when you need to. You don't have to tell them why. Just go have fun with them, or let them distract you.