Yeah the instructions should clearly say don't bend the flap back more than 90 degrees. I recommend closing it as soon as you remove the ribbon cable so that it doesn't get caught on something and break while you're working on the board.
What I do is run the antenna cable between my fingers to get it as straight as possible. Then I gently feed it under the cartridge slot slowly with the connector facing up. I stop at the slightest resistance and jiggle it back and forth to get it going again. Works every time.