You'll go crazy (or drive everyone around you crazy - arguably worse because in a "people profession" having people like you is a /good/ thing) if you focus too much on marks in med. It's just not realistic to learn everything, because it's actually impossible. Med is really a different ball game; the goal, in my opinion anyway, is to know enough so that you can back yourself to be a good junior doctor and to know things on the ward. There are people who I've seen slave away in the library during their clinical years... which means, sure, they might get a handful of extra marks on the written exam (big whoop in my opinion), but they're really wasting their time and the hospital's time otherwise.
My philosophy when it comes to med, and everyone has their own take mind you, is that I'm more concerned about clinically-relevant knowledge and experience rather than marks. Some might say the two come hand-in-hand, and they do to an extent, but there are certainly ways to go about increasing your marks without increasing your clinically-relevant knowledge and experience. To me, a few marks here and there aren't worth all the extra studying, I'd rather spend that time (if I am to study) doing something I enjoy, just as what interests me, or helping out on the ward, or seeing patients.
Furthermore, on a realistic note, marks don't matter all that much? No one cares about them after you get an intern job (unless you did a stellar job and won a bunch of awards, but even then...). As for how much they effect your intern application - not as much as you'd expect. Plenty of other things come into play, such as having qualities references, a solid CV (research, extra-curricular, etc.), a nice cover letter, and strong interview performance. So even if you did pre-occupy yourself with marks, you might be letting yourself down in those other areas.