Typical bell curve grading gives A/A- to the top 10-20% of the class. Most applicants have a few B/C's on their transcripts too. Are you surprised that medical school students are pulled from approximately the top ~15% of the nation's undergraduates? SDN is furthermore a self-selecting group where you see a bimodal distribution of the top students and the marginal students with relatively few in between.
As far as your situation, if working as hard as you can gets you to a B level then there is no reason to think that you will pull out A's in a post-bac. You need to take it slow (try a semester with just 1-2 science classes) and figure out what works for you. I refuse to believe that, utilizing the best study methods for your situation and putting in 110% effort, you cannot get over 85% in a class. Place that bar higher and expect better of yourself. You can do this 🙂