First — I'm really sorry to read this. Putting in months of prep and then seeing a 555 when your last mock was 665 is genuinely gutting, and your reaction makes complete sense.
But here's what I want you to look at objectively: your mock scores were 595, 615, 565, 615, 575, 665. That's a 100-point swing across your mocks. That level of variance is the real story, and a 555 on test day fits right inside that range — it's not an outlier, it's the bottom of your real band.
What that tells me is this isn't primarily a content or concept gap. You've demonstrated you CAN score 665. The issue is you can't do it reliably yet. The reasons are usually one or a combination of:
1. Pacing inconsistency — you're managing time differently from test to test, which shifts your score drastically.
2. Stamina and nerves — the real exam adds cognitive load that mocks don't fully replicate. If you're dropping accuracy in the second half of any section, this is your culprit.
3. Weak spots in specific question types — if your 565 mocks and your 555 actual have the same wrong questions clustered in the same topic (say, Data Sufficiency word problems or Critical Reasoning Inference), that's a structural gap you haven't fixed yet.
Before you book again, go back through your ESR (if you got one) and your last 3 mock error logs. Sort your wrong answers by topic and time-in-section. If the errors are random across topics, it's pacing and stamina. If they're concentrated, go fix those 2-3 topics specifically — not "all of Quant."
You're not as far from your target as today feels. Keep going.