I would let it go, and chalk it up to her other issues. It’s not enough to get into it with her. Knowing you on FB, I know your posts are not offensive or off putting in any way. There may be a whole host of reasons why she unfriended you, one of which may be envy that you aren’t dealing with the same issues she is. Some people on facebook do a lot of comparing other people’s posts/“outsides” against their own “insides”, without fully realizing we all have our own burdens to bear.