The 24-year-old, who moved to Lord's from Glamorgan at the end of 2012, took 9-34 in the second innings to finish with match figures of 13-103.
"I've had two very lean years which has hurt me," he told BBC London 94.9.
"I came here as a fairly big signing and expected to be doing these sort of things from the off. I am there to perform and I wasn't doing it."
The right-arm seamer added: "I came on a decent contract and wasn't performing so I was getting some criticism, and rightly so."
Media playback is not supported on this device
Harris took 21 wickets at an average of 38.33 in his first season at Middlesex but his second year in north London was more difficult, following a change in his action.
He took just 12 wickets at 55.50 in his seven appearances and returned to Glamorgan on loan in order to find form.
"We changed last year to be better - you want to be better and you want to improve," he said.
"Many things have been said that if I was half a yard quicker I'd be that much closer to hopefully getting an England call-up. That's what we went searching for.
"You could argue that we probably found it, but it was at the detriment to what I did naturally.
"I lost my height, I wasn't hitting the seam and I wasn't causing problems."
Harris has now re-modified his bowling action over the winter, culminating in his best bowling figures since his second first-class match for Glamorgan in 2007, aged just 17.
"We made a choice to go back. This is the type of bowler I have always been," he said.
"I was never going to be express but I'm hopefully bowling at a decent enough pace and doing a bit with it.
"I said all the way through the summer so far that I had been bowling quite nicely but there was still something to click.
"I got in a nice rhythm and I'm thrilled. It has been a long wait to get another 10-for."
Middlesex bowler James Harris says his career-best figures in the victory over Durham were "a long time coming".