Look at the mediation as a possible blessing. It is less stressful (and expensive) than going to court, and if you are having trouble communicating, it can be less stressful than trying to personally negotiate a compromise.
I agree that if he has an attorney, you should, too. If he doesn't, then it is really up to you. A lawyer can be good, though, if only to minimize the personal contact you have to have with each other during this process.
Try not to flip out too much over this. Just look at it as a step in the process, and take it a step at a time. It doesn't have to be a disaster.
Hugs,