+1 for careful measurements and slow squeezes of the lid.
Flip the lid upside down, and rest the edge of the lid on your sternum. Then grab the opposite side with both hands and gently pull it toward you. Like we've said, go real slow, especially if you are very strong. ;-)
If you can find a large, flat surface, you can also check your lid for true. Set it on a piece of cast concrete, or a large cut stone, or a countertop and see if it is flat, or it rocks. Not sure you can fix an out-of-flat lid, but you can at least save yourself the trouble of trying to get it to fit.
You can also twist your lid around on the bowl to see if it fits tighter in one position than another.