Sounds like you're trying to feed the board into the bit by moving the board. Generally a dado like this should be cut by running the router across the board using some sort of guide clamped to the workpiece.
The following video is on how to make a jig called an exact fit dado jig, which allows you to easily make perfect dados with the router. It is reusable. Next best way to make them is with a table saw, be even there you will find it requires a jig to reliably make the cut 90 degrees to the edge of the board.
Here's the video. Pay particular attention to how carefully he set the glued pieces 90 degrees to the longer guide pieces. And notice that he took pains to make sure the guide edge is dead flat. You will be using a half inch trim bit, maybe 3/4 inch long with a bearing at its base. Attached is a picture of such a jig and small mortising bit with half inch shank.