A properly built wheel relies on even spoke tension for its longevity. Uneven spoke tension is probably what caused your first spoke to break. Due to the fact that you have been riding for so long on a wheel with spoke tension that's all out of whack, what has probably happened is that you've put undue stress on some of the spokes (the tight ones), thereby decreasing their lifespan. Also, the fact that it's a wheel with a freewheel indicates to me that it is probably a fairly inexpensive wheel, which probably has low quality spokes, made of a poorer quality of metal than higher end (DT, Wheelsmith, Sapim) spokes.
So basically, it would probably be worth you either respoking the wheel completely, or buying a new wheel, because although you will be able to rebuild it, you are likely to have a "cascade" of spokes breaking one at a time.
Just sayin'.