How has Boxer had a difficult time adjusting to the new lifestyle of Manor Farm in Animal Farm? Despite all the hard work he does and the fact...

Boxer is probably the hardest working animal on Animal
Farm, and perhaps also the most loyal to the "leadership" and "vision" of what the farm
could become.  His "I will work harder" mantra shows that he is willing to put forth the
effort in order to benefit the farm as a whole, which could symbolize the ideal worker
under a communist regime.  "Napoleon is always right", his other quip, shows his blind
obedience to authority, which is another admirable trait according to those who abuse
power.

As far as adjusting to the "new lifestyle", I
suppose this might depend on to which section of the novel you are referring.  As the
novel progresses, the other animals seem to spend less and less time working for the
"common good", so Boxer is forced to take on added responsibility and burden, despite
his declining health; after a while, his health fails him and he's sent
away.

Throughout the story he is too uneducated to
understand what is going on around him, so he believes that just working hard and doing
what he's told will be a benefit to all.  This, unfortunately, ultimately leads to his
demise.