I think that psychologically not knowing when it will end is extra difficult. When I served in Afghanistan I knew pretty well how long the deployment would be so my expectations were set. If they kept adding extra months I'd imagine you start to feel like Yossarian.
Getting extended arbitrarily is horrible. The only thing worse is when they let someone else go home early, and force you to stay for no real reason. (Not even drawing straws or similar.) That genuinely broke me. Doing this to people is cruel, full stop.
(Fortunately, for as bad as it was for me, it did at least make it impossible to ignore my mental health, and was the beginning of a long, long road of treatment.)