It sounds like you're only a couple of weeks away from your next battle with the GMAT now, so I apologize if this is too late to be helpful.
For starters, it isn't unusual to see five or six SCs at the beginning of the verbal section, and it isn't unusual to see really, really long sentences that are almost completely underlined. It doesn't happen to everybody, but it's a reasonably common complaint. You might not be so unlucky next time around, but it's probably good to be prepared for this to happen again, just in case.
This might sound weird, but I think that the length of an SC question doesn't correlate very strongly to its difficulty level. The long questions are intimidating, time-consuming, and annoying. But they aren't necessarily harder, and they don't really require a radically different strategy than any other sort of SC question. Ideally, you're trying to efficiently find errors in the answer choices, whether the sentence is long or short. The longer SC questions tend to have tons of pronoun agreement, verb agreement, comparison, parallelism, and modifier placement issues--it's just that they "feel" hidden because the sentences are so effing long.
It sounds like you have a strong grasp of English in general and GMAT SC rules in particular, otherwise you wouldn't have been able to score above 40 on your practice tests! But the funky thing about the GMAT is that knowing the rules isn't enough: you have to be really great at recognizing them quickly on SC, even if they're buried in a long sentence. I suspect that you already understand all of the issues that I mentioned in the last paragraph, but you might want to spend some extra time making sure that you can recognize those keywords and structures instantly, wherever they may appear in a sentence. For practice, you might also consider cherry-picking the OGs and GMATPrep Question Pack for the longest SC questions right before your test, even if you've already seen all of those questions before.
I have funny feeling that you'll do just fine next time around. Good luck!
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