@mummy917
Probably some part of me is looking back at everything with rose tinted glasses on, but there were some good, happy times too. The entire relationship wasn’t unhappy.
Of course you have good memories! And there's nothing wrong with remembering them when the time is right. I have many wonderful memories of my marriage. I take them out and look at them from time to time, why wouldn't I? These memories (good and bad) are part of who we are. For you, for now, the trick is not to linger too long with them if they're causing you pain or self doubt. If so, remember they'll always be there but for now, put them on the top shelf of your 'memory closet'. It's been long enough for me now that I can look at those memories with a smile on my face. They no longer cause me pain or make me doubt my decisions. You'll get there, it'll just take some more time.
I’ve been thinking about the most stupid things, such as when he moves on, he will probably treat them amazingly as he will actually like them and want to be with them and it makes me feel sick.
My DS1 went through two horrible traumas within about 18 months. He almost died in a traffic accident then discovered his wife was cheating on him. He said the most important thing he learnt (and passed on to me after I left esDH) was "That's a tomorrow problem". Can you do anything today about the way Twat will treat your DC in the future? No. So, that's a tomorrow problem. Put it on the shelf with those memories. You aren't telling yourself to 'forget about that', you're simply telling yourself that you will deal with it when it happens, and not before. It takes practice, but it works.
I’m not sure if this is partly me reaching burnout too after the last few months and obviously firsts are hammering home just how much I’ve lost.
Could be. Or perhaps you're trying to deal with too many emotional things that need to be packed away for a bit longer. At any rate, it may not be a bad thing to be away from home. Try to tell yourself "I'll deal with things later, when I get home" and focus on the immediate 'now'. Don't rush home just to deal with them. You have plenty of time for that.
Over the last 18 months I've discovered that if I take things slowly, setting aside what isn't an 'immediate need' I can approach the hard emotional stuff more quietly and calmly. Before now, that would have been so against my nature as I was a 'get it ALL done yesterday, handle it now' person. Now I'm more a 'give it time to resolve itself, think about it later when you aren't frazzled' kind of person. I think I like the new me better.