Replacing a bridge on an acoustic is not the most difficult task in the world, but not the easiest either. I have to ask though, are the strings cutting into the actual bridge itself, or the saddle? The bridge is most likely made of rosewood, and in nearly all cases has a white bar (saddle) that lies comfortably in the bridge. It is this white saddle that is most often gouged into LONG befor the bridge is, considering that the saddle is at least a sixteenth of an inch higher. If its just the plastic saddle thats shot you are in luck. remove the strings, wiggle the saddle out, take it to a guitar shop and ask for a bone replacement. They will sell you one for about 6 bucks. You are then faced with the task of cutting and shaping it to the same height as the original. A dremel is very handy for this. Its almost a definate that you will have to do this shaping as odds are slim you will find an exact replacement saddle. If it really IS the bridge thats gouged then you have a far bigger project ahead of you.
Sidenote: the gauge of the strings you use is a preference. The fact that they cutting in because they are a light guage is simply speeding up the inevitable with any plastic saddle. Fairly common, and easy to upgrade as mentioned above.
