If your router loses stability when trying to route narrow edges, a jig made from a piece of fence post can hold it stable.
When trimming solid-wood lipping flush on the edge of plywood, I found it difficult to balance my top-heavy plunge router. Here’s my stabilizing solution: I took a scrap piece of 4 x 4 fence post and jointed two adjacent sides flat and square. Then I cut a 3/8″ x 5/8″ rabbet along this edge to provide clearance for the overhang I leave on my edging when I glue it in place. By clamping my jig flush with the surface of the lipping, I add almost 3-1⁄2″ of support for my router base. Trimming edging is no longer a balancing act.
