Seems like your studying really hard, all the best for your step I.
There are two kaplan simulation one proveded by ECFG when you apply for Step I the send that CD (three block only) (download from here
Multiple Choice Tutorial and Practice Test Items 12MB ) most of us did that 15 days before the exam so that we could know where we are.
And the next one is you can buy or ask any of your friends who have KAKPLAN simulation CD which is just like you are giving a real exam.
I can't say NBME alone but
will give you your approximate performance in the real day. But believe me real day is real day you can't imagine.
How many of you are studying, at least you should have 1-2 study partner alone you can't prepare well. If you are stuying alone find study partner or you can make online partner too.