I’m so sorry you are going through this. I found out my husband was secretly using cocaine (I had no idea or ever thought it a possibility). We had already separated due to the arguments, violence and basic chaos that our lives has become (and I didn’t know why).
The nice bits you describe are always the first things you think about and it’s like a form of torture but it’s a grief and like you say the darker parts which make us miserable, because we don’t have that reliable person do take more weight at some point in the grieving process.
It’s awful. I wouldn’t wish this emotional whiplash on anyone.