The 22-year-old centre-half put in a solid display against Rangers on Wednesday night in the UEFA Champions League, after some shaky performances of late.
"I don't know why I wasn't performing well earlier in the season," said the Northern Ireland international.
"Last year I had a stop-start time and got one or two injuries when I was in the side.
"I wasn't at my best for about six months. I was making decisions and thinking 'I would never have done that in the past'.
"I have tried not to worry about it too much. I just got on with it and hoped everything would come back."