Hi Joey
I think heat treatment is the only way to relieve the stresses. Since most plastics have a fairly low melting (or at least softening) point, dunking them in hot water should do the trick.
You can either heat the material, bend it hot then let it cool, or bend it cold then heat and cool it to relieve the stress.
Some experimentation would be needed to arrive at the right temperature/time, but the easiest to try first is to bend it cold, immerse in boiling water for "a while" then allow to cool naturally. It may be advisable to have some means of holding the form whilst it cools to prevent any unwanted sagging
Good luck, and don't forget to tell us how it goes!