How you go about addressing the problem really depends on where you think your deficits are, and what you have done to prepare in the past but doesn't seem to be working for you.
If you are having trouble with how to approach the questions, pacing, and finishing the exam, then have you been doing enough practice questions? You say you have never been great at taking exams. But it sounds like you have passed all the rest of your shelf exams this year, so I'm inclined to think that it's less likely to be a just problem with the exam format/familiarity with question types.
If this is the only shelf exam you have ever failed, and you failed it twice, is it an actual lack of content knowledge in ob-gyn? What resources have you used to learn the actual clinical content? How did the rotation itself go? Did you study the material and gain sufficient clinical experience at your particular rotation site?
You may want to talk with some of your classmates and find out which resources they found most helpful for the ob-gyn shelf.
During my MS-3 year, I used the following during my ob-gyn rotation:
- Obstetrics & Gynecology, 5th ed, by Beckmann et al-- a general textbook for med students and primary care residents, used it for background reading throughout my rotation. Also comes with a CD with review questions for each chapter, they are not really shelf-type questions, but they are a good review of the material itself to check your comprehension and retention of what you read.
- Blueprints Ob-Gyn-- one of the better titles in the Blueprints series, in my opinion. Good for quick review of the major topics in the week or two prior to the shelf exam. I also liked browsing Blueprints as a topic overview before delving into more detailed reading in a textbook.
- Appleton & Lange Ob-gyn question book-- the questions are hard, harder than the actual shelf exam, and the explanations are quite detailed. I thought A&L was better than Pretest for Ob-Gyn. (A lot of the other Pretest books are good, just not the Ob-Gyn one)
Good luck to you!
ps- if you are worried about the Peds shelf, I thought Blueprints Pediatrics was excellent, and Pretest Pediatrics was good for practice questions.