I've gone through about 3 or 4  gas struts that  help open the door on my MF 3545 tractor.    It's the type of cab that the door is  practically the entire side of the cab,  and when the wind is strong,   Every bit of effort is needed to open the dang thing.   One time in  40+mph wind,  I had to turn the tractor the other direction just to get the door open.

Anyway,   I've replaced it with both  Agco boxed  struts, and aftermarket  with  about the same result: Shot in 2 months or less.  Last one  lasted 2 weeks before it gave up.  (the  gas assist part)  

I will say when the door opens,   it hits the end of the stroke pretty hard.. it's a heavy door.   Is this  pounding the piston out? 
 Is there tougher cylinders to consider here?