hi adma....sorry to hear that; i will not try to point what you did wrong but i really wanna help you because my situation with step 1 was so similar to yours.....here is my advice:
18 months preparation is way too much; you will have to do it in a very condensed time(4-6 months but studying 10-14 hours a day, 7 days a week); i will give you some tips and it might help you.....forget about what just happend; keep your head up and move forward(i got an 79/195 for step 1 which is not a great score but i knew i have to work extra hard on CK to get a better score)
step 1: pay thh exam fee and choose june/july/august as the period when you wanna take the exam
step 2: go through all kaplan books in maximum 4-6 weeks(do about 1 book per week and don't look at video's anymore)
step 3: after going through kaplan books, do the kaplan qbook(850 questions which can be done in maximum 2 weeks, aka by doing at least 2 tests per day and reading ALL the explanations given for right and wrong questions; again 10-14 hours a day of study)
step 4: buy usmlerx for 1 month; you get 2500+ questions which are not as hard as kaplan qbank or uw qbank but will make you very confortable about how to approach questions; by now you should be able to do 3-4 tests per day because you will see how your procentage of right questions will keep rising comparing to kaplan qbook; in this way you will be able to fill a lot of gaps in your knowledge; 2500+ questions can be done at least one time in less than a month(depending on how much time you still have from your subscribtion, look one more time on your weak areas and do the wrong question)
step 5: buy kaplan qbank for 1 month; you get 2200 questions which should be done at least 2 times in 1 month(you will see how confident you will be and at this time and some of the questions will seem to be ridicuously simple); after going once through all teh questions take an NBME(take form 5 or 6 but not 7).....if you are happy with the score, take the sooner date available on prometric and go to the exam and in mean time take NBME 7; if you don't like the score, buy UW for another month(the only problem with UW is, since you already bought it now, innevitably you will see some questions from your previously subscribtion and that's the reason why i did not sugest you to buy it at step 5-i suggest kaplan); in this month you will just built more and more confidence and getting 100% ready for the real deal; after going once through the uw questions, take NBME 7; i am pretty sure that at this time you will get a decent score(above 200 which will give you enough peace for the real exam); from NBME 7 until exam day do uw questions(second, third view, wrong ones, etc)....ideea is to do something EVERYDAY until the day before exam; at this time you can take the free NBME just to keep you in the shape for the next day: a 75% and above is a good index that you will pass the exam.
if you do what i just wrote above, i am pretty confident that you will get a good score and summer time will be happy time....you might ask why to do so many questions? i did not do so many for step 1 and i burned myself; as many as you do, better it will be on the real exam.....is all about practice, practice, practice.....
i hope my advice will help you.....don't look back and don't regret anything. look forward and try to improve what you did wrong in your first attempt....i wish you gook luck and all the best and in the summer i wanna hear you scream when you will get yor score from usmle....cheers!