Seven years bad luck

My mosaic mirror has been hanging on the wall for the first time ever for about a month. It cracked. The mirror part, no the grout. My guess it that it wasn't designed to hold it's weight and there were stresses in it. I had epoxied the glass to that particle board stuff with the holes in it. I guess I didn't realize epoxy would not expand and contract enough to join these two different materials together. So, my plan is to either place a mirror over the cracked one inside the recessed area, or try to chisel out the cracked part. The first one sounds easiest, by far.