The 30-year-old was targeting a third appearance in the competition at Medinah but poor form in the second half of this season saw him fall out of the qualifying places and persuaded Davis Love III to overlook Mahan as a captain's pick.
"It was hard - harder than I thought," Mahan revealed. "Golf is hard right now. I just can't deal with it all. It doesn't have much appeal. It feels empty.
"You need to be mentally and emotionally invested in each round and, if you're not, then it's hard to compete out here. I'm not even technically good right now."
The former world number four also claimed that he will not be watching September's contest.
"I don't feel good saying that, but I think it would be hard to sit and watch it. I'll watch the match results each game, at the end of each day," he added.
The Ryder Cup will begin on September 28.