Harper started his first top-flight match since April 2011 in Monday's draw at Everton.
And the 37-year-old, who replaced the injured Tim Krul in the Toon goal, admitted: "I didn't think a night like this would come again. It looked a long way away.
"It was probably a little bit my fault as well. When Tim started last season, I admit I got the hump and let my standards slip.
"That's probably the reason I went on loan and wasn't involved.
"But the first morning of pre-season the manager asked to see me. He said, 'Have you still got it?' I said, 'Yes'.
"He told me if I showed him I still wanted it, there would be an opportunity. So credit to him."