Hi. That sounds a lot like what I was dealing with. Some very wonderful, special moments or times, and then without warning, he'd snap, belittle, criticize, yell. Or get really dark and negative (I called this his toxic state). Also periods where he just dismissed or ignored me. I too walked on eggshells, trying not to trigger any of this bad stuff. But the key word you say is "unpredictable."
It took me many years to learn a very hard lesson that this behavior in him was not something I could control with my behavior. There was no bright sunny action I could take or way I could be in order to prevent these drastic changes of weather, so to speak. This is because these behaviors stemmed from deep inside him, not from me. My issue of course was over-care and trying to tiptoe around him - it played into his negativity and made it worse--fueling a lack of respect in him for me. But ultimately, he is in charge of these behavior choices and you can't prevent them.
You say you are feeling emotionally exhausted - I get that!! I found this dynamic exhausting too. It might be worth investigating some things...might he be depressed or bipolar? Is this bad enough to be emotional or verbal mistreatment? I don't want to diagnose anything or label anything for sure...but the dynamic you mention is so familiar to me and is also so frustrating because the good times are so good I always thought why can't they stay; why can't I fix this?