I'm so sorry, my partner was actually INFJ. I just put a random one there.
My INFJ issues were related to perfectionism.
We went to therapy together, and it helped.. would reccomend. Therapy is a great safe place to ask them "dumb" questions about their behavior and thoughts.
If you're not at that point yet, keep researching about their brand of unhappiness and see if any suggestions come up.
Can i ask what improved the situation ?