I’m sorry that’s happened to you, and I can totally understand why you’re feeling disappointed. Rest assured, there are many ways to work through your plateau.
I don’t know much about your specific situation, but I can tell you that most people hit a wall for a few reasons:
You either have an accuracy issue or a timing issue or a test-stress issue.
Accuracy: you’re not able to answer questions correctly, even if you had tons of time and no stress. This means you don’t understand the material. This is probably because you’re not focused on learning and you’re more focused on simply testing. Testing is attractive because it feels productive and you have the chance to score well and then you get to feel accomplished and stop practicing. But unfortunately, taking practice tests when you haven’t studied the material is like trying to learn to fly a plane by stepping into a cockpit rather than having someone teach you how to fly. Focus on mastering the concepts that will be tested. Practice problems, then try testing.
Timing: you could answer questions accurately if you had lots of time. This means you need to get faster, which you can do by identifying the question types that take the longest and practicing those. Then start taking the test with 110% of the time, then when you get comfortable there, move to 100%, then move to 90%, etc. If you can successfully answer the questions with only 90% time, then you’ll be set.
Test-Stress: you could answer the questions accurately, in the appropriate amount of time, if you were at home, calm, and well-rested, in an isolated environment. The solution to this issue is a little harder, and it could be a deeper psychological or psychiatric issue. But to the extent you can address the issue yourself, look into some of the Test-Stress literature, software, and meditation programs out there.
Identify which section you’re having the most trouble with and begin there. Let me know if you have any challenges.