Yes, Sylvia was tragically plagued by sadness throughout her life. In an interesting coincidence, I wrote about Sylvia in the next installment of the series :) You might like that. But yeah, it's a terrible story. I got to feeling very down while writing about her. More so than any of the other figures in this series...
I've heard it said by some that Plath didn't really intend to kill herself, that it was a cry for help and she meant to be found (there was a man who called there everyday and she meant for him to find her while still alive), sadly, the evidence and the way she isolated her children's rooms suggests otherwise.

Anyway, yes, you're very right. Both Van Gogh and Plath (as well as many others, of course) only became admired artists and were considered geniuses after their suicides, which is a real shame.