I would remove the borders, and start over. What I do (if I do not have a surface big enough, can't use the floor anymore), but I now do it for all my borders is:
fold your quilt in half, finger pinch a fold in the exact middle. Do the same with the border. Match the two tiny folds and pin from center to end. Then sew from center to end. Do the same with the other center. Use a walking foot also. It helps feed the fabric evenly. Also try starching the border to make it stretch less before you start.