The midfielder has made 17 appearances in the Scottish Premiership for Ronny Deila's men this season.
The 23-year-old has made 17 league appearances during his first full season at Parkhead, but conceded that he has not managed to perform well enough on a regular basis to show why Ronny Deila brought him to the club in the first place.
"There is no denying there is a huge mental aspect to being at Celtic and the combination can be challenging," he said, as reported by The Scottish Sun.
"There are times when I haven't coped as well physically or mentally.
"But it's about learning and this is my first full season with Celtic.
"I'm learning more every day about how to accept being a Celtic player and everything that comes with that."
Despite his admission that he feels that he has not been up to standard, Armstrong - who has not received many chances in his favoured position of central midfield - has managed to show the kind of glimpses of quality that suggests that he has got the potential to prove a success during his time with the Hoops.
He has scored three goals and bagged four assists in the league to help the Bhoys maintain their position at the top of the Scottish Premiership.
And while he admitted that he has so far struggled to make the grade at Celtic, he has certainly got the talent to show those who doubt him that he can establish himself as an important member of the squad in the future.