Your kind and wise words make me feel better!
I don't think there was any love interest, she used to send me photos of her husband and making plans to meet her husband.
I don't know for sure if she has bipolar but I do know that a doctor diagnosed her with bipolar and she was taking medications. She has been stable for like 2 years now without medication so I really don't know.
I agree that the best thing is to move on and let go but without closure I find it extremely difficult. I'm not someone who makes friends easily so for me that having a sudden very intense friendship was a big deal. Then without any explanation its gone. This is still obsessing my thoughts years later.
The most realistic scenario I could think of is that our friendship started during a manic phase, so for me it was the normal her. But for her, she was not herself.