Are you certain that the situation you are having is actually a burn and not a "scalping" of the laminate? A burn or marring of the laminate should occurr along where the bearing is guided. If you are having problems up by the joint, it sounds like a scalping. There is a picture of this common problem in my book. It happens for a couple of reasons. First. if the edge of your countertop underlayment is not perfectly square, and is actually backcut, the bearing will guide the cutter closer than it should go. Another cause of scalping is when the trimmer bit is set too low and you rock or tip the trimmer. The bit should be set just deep enough so the cutter is contacting the top sheet (and maybe a skosh more) and that's it. And yet another reason that scalping happens is that some laminate trimmers are not made with as much precision as they should be and the base is not square to the collet (which holds the bit).
I know how you feel about being nervous about trimming edges. I've been fabricating laminate tops for almost 20 years now and I always get a small anxiety attack when I put the trimmer to work. A future issue of my newsletter will have an article about what to do to salvage your countertop after you've scalped an edge.
Hope this helps you- or anyone else who may be having the same problem.
Herrick Kimball Editor- Professional Kitchen Craftsman newsletter email@example.com (for details, or call 315-497-9618)