Where is the balance of wanting a good grade in a class and not stressing so much over it?

I have always been a good student.  Up to this point in my life, I have only made one B.  I don't say this to brag but to help you see where I'm coming from.  This semester I have one class that is kicking my tail.  I made an A on the first test (which was hard), but I just got my first paper back that I thought I did well on, and I made a 75.  I am so discouraged.

I say that I would be okay with a B, but the truth be known, I would be quite upset.  I want an A.  But I don't want to be stressed out all semester for one.  Ugh!  Where's the balance?  I want to think long term and see that I need good grades, but all I see is today...and today I'm tired on trying so hard all the time.

Christen