how did napoleon trick the animals into forgetting how hard life was for them

Napoleon uses false propaganda and fake statistics (fake news) to make the animals believe that life is better than it really is. His statistics successfully fool them into believing that they're better off under the pig's watchful eyes than they were with Jones.

Sometimes the older ones among them racked their dim memories and tried to determine whether in the early days of the Rebellion, when Jones's expulsion was still recent, things had been better or worse than now. They could not remember. There was nothing with which they could compare their present lives: they had nothing to go upon except Squealer's lists of figures, which invariably demonstrated that everything was getting better and better.