The England winger signed for his home-town club in the summer, but he failed to live up to the lofty expectations placed on his shoulders.
The former Middlesbrough man concedes that he needs to improve next term and he told the Northern Echo: "It's been the worst season of my career as a whole, I think.
"I actually haven't been that bad on the stats side of things, but performances haven't been what I've wanted.
"If you go through goals and assists, I haven't done that bad but performances have been sometimes. As a whole you look at yourself and you're honest with yourself.
"I think in general, everything hasn't gone well. We've got to look forward to next season now. It's all we can do."
Johnson also spoke about new boss Paolo Di Canio and backed the Italian's perceived hard-line when it comes to squad discipline.
"He's come in, and managers have new things that they want to do," he said.
"The gaffer is big on discipline and if it means us getting more wins, I'm all for it.
"There's nothing worse than losing game after game after game after game all season. I haven't been used to that in my career, so it's been difficult.
"It's been poor, very poor, I think. The table doesn't lie. I think the only thing that we can say is that the only way is up for us now. We can't get any lower to be honest. We survived, and now next season all we can do is progress. That's the positive way of looking at it."
The 25-year-old is looking forward to next season, but he admits he cannot tell if he will be even with Sunderland.
"I'm looking forward to next season and I want to improve, as far as I'm aware, I'll be here," he said.
"Over the summer you never know, it's always up to the manager. Time will tell what happens."