Could you help?
the hunger games 2 chapter 2
And the idea of losing him forever, my best friend, the only person I'd ever trusted with my secrets, was so painful I couldn't stand it. Not on top of everything else that had happened.
I don't catch the meaning of the last sentence at all. Even after I search "on top of everything else" in the dictionary, I feel still confused.
What's the meaning of the last sentence?