It would be unnatural if you weren't nervous, but hopefully some of this will help: one of our sandans was using his class to help kyu students due to test to brush up/clear up questions on techniques on their tests. You could tell as he progressed throught the class that seeing some of the higher kyu techniques the 5th and 6th kyu testees were getting stressed. At the end, the instuctor made some general comments about testing, and that the lower kyus shouldn't worry that they were not at the level of the higher kyus; "we look for different things out of each level," he said, "for 6th kyu, if we (the grading committee) can be reasonably sure what technique you are doing, that's about what we expect..."
You are probably being hard on yourself based on the levels you held in other MA, expecting too much too soon. Your sensei must think you are at least at (probably more than, hence the 'wear your rank' comment)the level he wants you to test at, and that you are a positive addition to the dojo, so he wants you to progress so you can share more with your fellow students...good luck!