Efe Ambrose believes he has bounced back from a number of big game errors thanks to the support of his Celtic team-mates and manager Ronny Deila.
The Nigeria international feels he now has the strength of character to prove he is a top defender.
"I believe those days are past now," he said. "There is a new beginning.
"I'm just focused to keep doing what I'm doing right now, which is keep my head cool, concentrate and to do the simple thing like the gaffer says."
Ambrose was heavily criticised after errors that allowed Fenerbahce to come back from two goals down to draw in their Europa League draw with Celtic in October.
And summer signings Dedryck Boyata and Jozo Simunovic have often been preferred to Ambrose in defence for the Scottish Premiership leaders.
"It's part of the game - it's part of life," said the 2013 Africa Cup of Nations winner. "You get criticised, but the most important thing for me is how you get back from that.
"During that time, the team stood by me, they helped me out and they were there to pull me out of it.
"The gaffer has really helped me as an individual and the team as a group.
"The most important thing is I never give up. I always tried to do my best even when I was playing and even when I made a mistake.
"I tried my best to see if I could come out of it and help the team. The most important thing is the team first, not just me or any individual."
Ambrose has been back in Deila's plans recently and the team have kept three clean sheets in the last four matches he has started.
"I cannot say I'm the best, nobody can say they are the best; we keep learning, football is all about learning," added Ambrose.
"Now I don't try to play out on my own by taking risks. The manager doesn't want me to take risks.
"Like a midfielder I tried to play, I tried to do too much, but now I'm trying to put that behind me.
"He has helped me to improve my game and I need to realise that defending first you need to concentrate to do the simple thing.
"Defend first - anything else is a bonus."