welcome to the de-lam club spinfly, plenty of us have that problem.
A sail maker cuts shape into the sail to give it effect. As the wind blows he wants a material that works to hold his design shape. Laminating Mylar onto Dacron increased the stability of the material resulting in better performance. It is only necessary to laminate one side. If the Mylar separates off the Dacron, performance of the sail drops sharply, and increasingly as the wind increases.
Removing the Mylar won't help, especially if it is still limiting the amount of stretch to some extent.
I don't know of any practical solution; if there is one out there I would really like to hear about it...
Depending on conditions (not too windy) the sail probably good for cruising, especially downwind