I'm not sure I understand the difference between "being" and "existence"
"The paradox of human life is precisely that one tries to be and, in the long run, merely exists." I think what she means is: if you have your life ahead of you, you can still choose your destiny (you can be something). However, having lived that life and looking back at it, history is fixed and there is nothing to choose when it comes to the past (it merely exists).
I took "being" to include being somehow immersed in the "far beyond" once the desired lifestyle is met. Like realising you're in a dream without eventually waking up.