The England flanker only returned on 30 September, after initially thinking he would be out for just six weeks after injuring himself in October 2015.
"It was constant setbacks, so mentally that was difficult," said Clark, 27.
"Through all your setbacks you do learn and I do feel a bit like I've taken a lot from it despite it being such a negative experience."
Clark twice had surgery on the problem, before playing his first competitive Northampton game since May 2015's play-off semi-final defeat by Saracens two weeks ago.
But the former Leeds Carnegie man has said he has "not been anywhere near happy" with the level he has been playing at since his return.
"I'm an intense person because I care," Clark continued to BBC Radio Northampton. "We get one shot at this career and I'm not willing to sit around and do a half-hearted job of it.
"There are times you have to relieve that and I probably need to be better at switching off but that's what makes me, me.
"I don't think I'm good enough to chill out and let it happen."
Northampton Saints' Calum Clark says it was 'mentally challenging' during his 11 months out with a shoulder injury.