I understand why people are saying that you're a troll, but having been in your position, I know you are not. I've had conversations which basically boil down to the original post with a few friends of mine in real life who have also been in the same situation. No, it's not wrong to aim high, and there is a psychological factor of having a 4.0 which makes it really suck when you lose it. However, it's far from the worst thing that can happen and I'm sure a few weeks from now you'll realize that "today was the worst day, I'll never forget it" was going wayyyyy overboard. Put things in perspective and cut the hyperbole.
One B will not make a difference. The medical school admissions process is not as random as it seems. I have trouble believing that sometimes, but I know that is is true from what I have heard from people with more experience in the process. An exceptionally high GPA and MCAT score gives you a ticket to consideration at top schools that few other qualifications can. Whether you have a 4.0 or 3.95 doesn't matter, it still earns you that ticket. Once you've established your credentials quantitatively, it comes down to the qualitative factors to decide whether you get in. Nobody is going to give a candidate with a 4.0 a full ride over a candidate with one B - the decision between those candidates will be made on things other than their numbers.
The one thing to watch out for: when you have a 4.0, it provides a ton of motivation to keep getting perfect grades in other classes. I got an A- in a class which I know I would have got an A in if I had taken it before I lost my 4.0. I'm not really upset about it, because my GPA is still awesome and an A- won't hurt me, and when it comes down to it I'm not sure it would have been worth it to put in the amount of work in that class that would have been necessary for the A. But just watch out and make sure that you don't get lazy, even without realizing it.
In terms of positives, I've heard from engineering friends as well as contacts who hire engineers that within that field, a 4.0 is sometimes viewed with suspicion because people with perfect grades tend to be the type of people who don't relate well with others. That's an unfair generalization, but outside of medical school admissions, having a very good but not perfect academic record might actually be ideal.