The Scotland striker made just 11 appearances last season under five managers, including Dougie Freedman during an uninspiring loan spell at Crystal Palace, and netted only one goal. Goodwillie re-signed at Tannadice on a six-month loan deal on Tuesday less than two years after moving to the English Premier League in a deal worth at least £2million.
"It's been horrible because when you're not playing it's hard to stay focused and stay happy," the 24-year-old said. "You try your best but inside you know you're not happy."
He added: "I worked hard down there and just never really got the opportunities. The change in manager has been a big factor. How can you get a manager to like you when it is changing so often?
"There is a lot of competition and maybe the boys who were in ahead of me were doing well. There is no point changing it if it wasn't broken and I just never really got a chance. I don't think I should have been playing every week or anything like that but I just never really got a chance."
Goodwillie has had some difficult times off the park since making the grade at Tannadice, some of them self-inflicted, but he appeared a more mature individual when talking to the media at the club's St Andrews training ground.
"I don't know if it has made me stronger or not but it has probably made me more professional," he said of his spell at Rovers.
"It's been frustrating but it's been good as well because I have learnt a lot. I have not played as much but I have learnt a lot and come back a wee bit more mature."
Goodwillie netted 19 goals in his last full season at Tannadice and will forever have a significant place in the club's history following his goal in the 2010 Scottish Cup final win over Ross County. And he admitted the move has already begun to rejuvenate him.
"I got a wee call and asked if I would fancy it and right away I got a little buzz and I thought I would love to get back playing regularly and enjoy football again," he said. "It was an easy decision."