This is such a good question. I'm struggling badly with this one.
Four years ago my H relocated to the guest room, then ran off to the mountains for months at a time, had a couple of EA's, and eventually decided he was returning... and then asked if I'd be ok with that (not that we were getting back together, mind you; he just didn't want to live in a tent anymore). It was more than a year after that, that we actually started to reconcile in any shape or form.
My H has done the heavy-lifting work of lugging his shrinking conscience from terrified to guilty to actually remorseful. He says he's not that guy anymore, and truly he looks and sounds different. But how can he prove to me that he's trustworthy? By not doing anything bad? How could he prove a negative? H thinks he can just live with me, be a decent guy, and try not to set off my (multitudinous) triggers. And hope that one day, it'll be enough.
When will it be enough for me? Well, I'm honestly delighted to recover a relationship with H (as he is now). But in the back of my mind, I can't rest easy - it will happen again, my brain tells me, just wait and he'll do it again. He'll dissociate and drift, just like he did before. Some weeks back, when H (accidentally? mindlessly? on purpose?) really hit a trigger with me, I told H I can't trust you, and H reacted like I'd hit him. "Do you know what it feels like, to know you don't trust me?" H asked. Then he withdrew for days, in a stew of remorse. I could've yelled, "you don't know what it feels like to live through your partner's MLC...". But this could become an endless cycle - H breaks my trust, I can't trust him so I berate him, he can't let himself be vulnerable to me... so he will break my trust. It's a koan - what is the sound of one hand trusting? So... this is what I realize: independent of whether H deserves my trust or not, I have to learn to put aside my (realistic, dammit!) doubts and to trust. It's like walking on a fractured limb. I don't know how to do this. But I know I must.
"You have a right to action, not to the fruit thereof; shoot your arrow, but do not look to see where it lands." -Bhagavad Gita