Have you pulled the tank neck out of the grommet first? It might be a pain getting to the upper clamp, but there's 2 clamps on the hose connecting the tank to the filler neck. If you can get the upper clamp loose, you can twist the the neck and pull it out making it much easier to intall the new grommet.
Alternately some guys have used twine in the channel like installing a windshield, pulling the lip in from underneath.
