Welcome to the forum Steve. You obviously have a guide on the one side so that is the side that is good. In the meantime the router is bouncing around on the other side. It could be that you are feed in the wrong direction. Going one way, the force of the bit in the cut will push the router against the guide. Going the other direction it will pull it away from it. Sometimes it is also hard to control the bit when routing across the full face of the bit. My suggestion would be to cut the piece off with a saw and then take a small cut along the edge to clean it up. Otherwise I would use a guide on both sides of the router so that it can't dance around. Also, if you are starting the cut as a plunge make sure you are using a plunge cutting bit. Some aren't designed to plunge.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.