If you are restringing your shades every year you are 'making the strings too tight' when you install them.
I have had day/night shades on all of my MH's. 20 years combined And I have only had to restring one of them. And that was because I dropped something on the bottom plastic screw and broke it.
And I'm a flipper of day/night shades all day long everyday. I use them a lot. My strings are not real tight.
Check how tight your strings are next time you repair them. Loosen them up a bit and they won't keep breaking. :C