Sorry.. I disagree. Using a comma before "and" when it's between two clauses that are both short and related is generally considered not necessary. You rarely see it in creative writing. Normally you see them in long lists of clauses or between clauses that are about different things.
The clauses "mistake in her choice of partner" and "the inevitability of impending problems" are related and both of them are factors that "filled her with strong anxiety and fear". That's why I think it's a correct sentence as long as that one comma is added. And the reason I prefer it to the other option is that it is shorter and more concise. These are Visual Novels and much of the communication is already there, in visual form. Dialogue should be there just to supplement the visuals presented and not to replace them.
That's my opinion. But English is English, and there's many ways to do it. It can also depend on the region as well. US English is quite a bit different than Canadian English, for example. Just as Canadian English is different than British English, lol.
I'm actually thinking about taking some night courses in creative writing to improve my writing. High school English classes were a long, LONG time ago!
i didn't say the clauses should be separated by commas, but that's the only way there should be a comma after "problems", imo.
maybe we see different clauses. consider this:
A) "Realizing the mistake, she was filled with anxiety." -- the comma here is needed
B) "Realizing the mistake, filled her with anxiety." -- the comma should not be here because "filled her with anxiety" must be paired with something else.
if we switch the clauses in B, we get "Filled her with anxiety, realizing the mistake." that sentence makes no sense.
but if we switch the clauses in A, we get "She was filled with anxiety, realizing the mistake." making perfect sense.
so the comma should not be in B. it should be "Realizing the mistake filled her with anxiety."
i'm not claiming i know this to be correct, but this is what appears correct to me