The latch on my Andersen sliding door keeps breaking

There could be a few reasons. One could be that the door lock needs to be aligned to the keeper, since you're forcing the thumb lock to engage. Another could be that the inside mortise lock is broken. I would closely examine the hardware on your door that interacts with your thumb lever and see what kind of condition it's in. Try removing the thumb lever and activating the mortise lock manually, too.