I have an epoxy countertop that I did myself almost 2 years ago and it still looks fabulous. Every once in a while, someone puts down a plate or a cup with a harsh bottom edge on the counter and it leaves a dent. Sometimes quite deep! I find that during the daytime, when the room is warmer, the dent disappears over the course of a few hours. I'm wondering now about the possibility of you using a heat gun to warm up the area, not holding it in one place for too long, to fill in the gap a little bit without having to use any sanding at all and still leave the glass finish. It's worth a try and can't be worse than scratching it up even more with sandpaper.