I read her biography a few years ago, in spite of being eulogised in at least three songs, she wasn't really happy in either of those marriages. Eric Clapton in particular was awful to her for much of their time together, so much for Layla and Wonderful Tonight. It was a source of unhappiness that she was unable to have children.
Letters are personal, often the writer is expressing feelings at that moment in time and not for the eyes of the world to judge. I've probably read letters of the long since dead, kings, queens and other well known people but not sure when some of the people involved are still alive.
There are lots of beautiful iconic people from those times, who haven't aged so well, Bridget Bardot for example. Patti Boyd was lovely in her hey day, but they are just mere mortals like the rest of us, it's the passage of time it affects us all to a greater or lesser extent, depending on all the booze and drugs individual genes, it's hardly surprising.