Take a deep breath, Shmpoosters, because this is one big paragraph:
After wondering if his protagonist will marry or not, the narrator explains that he can't determine her fate. It's another little taste of existentialism: "To be frank, I am holding her destiny in my hands and yet I am powerless to invent with any freedom: I follow a secret, fatal line. I am forced to seek a truth that transcends me" (2.39).
In other words, Rodrigo S.M. is experiencing the angst that comes with the fact that there are consequences to his freedom.
Also, truth, for existentialists, is subjective, meaning each individual decides what is true for him or herself. Problem is, even this "truth" can change, so there's objective, universal, and permanent truth.
If you think it's hard now to decide what to have for lunch, imagine how much trouble you'd have if you believed there was no such thing as absolute truth.
Okay, so, if it's so hard, why does he want to write about this girl?
Apparently, he thinks that there's something sacred about her poverty and seclusion, and he wants a taste of this sanctity.
We have to wonder if she experiences her poverty in quite the same way.
He says that he writes because he's "desperate and weary" and he can "no longer bear the routine of [his] existence" (2.39).
This is kind of like when college freshman stay up all night saying things like, "You guys, you can never stop breathing. You have to breathe every single minute."
Anyway, it seems like poor-little-rich-boy Rodrigo S.M. suffers from his own kind of lack and emptiness.
If you have the sense that so far, nothing has actually happened in the book, you're right.