The 26-year-old says the support of his family plus his determination and success with Belgium helped him maintain his mental strength even when things were going badly for him and the club.
Fellaini was former United boss David Moyes' only signing of the 2013 summer transfer window when he arrived at Old Trafford for £27.5million from Everton.
He did not score a goal in his first season at the club and failed to discover the form he had shown under Moyes at Everton, but he has found his feet under Louis van Gaal this term with some impressive performances.
Asked if felt he was made a scapegoat last season, Fellaini said: "Yes a little bit. That is a difficult question. It is difficult to answer that.
"I didn't lose faith in myself. In football you have to be strong in the head. The mentality is important and I do have this quality so I can keep going.
"My family helped me through it. I was also in the national team for Belgium at the World Cup and I played well and the team also played well. For my confidence that was important. I came back after the summer confident.
"The manager said to me I would have to fight if I wanted to play. I did fight and trained hard and that was it. When I got my chance I tried to show myself and tried to do my best.
"Last season was a difficult season for me. The World Cup helped me a lot. The manager of the national team and the players helped me and it was a great experience for everybody."
Fellaini said his current form is partly the result of recovering his fitness after an ankle injury - but that there is more to come.
Speaking at an event organized by Manchester United's global partner Nissin, he added: "When you are playing you are always happy. So I am happy at the moment. I have worked hard and will work hard to play every game.
"I am fit now and that is why I am better. When I am super fit I will be better I am sure. I was injured earlier this season, they said I had to have surgery but I didn't. It was a bad injury but now I am better and I can look forward.
"At the moment the manager is using me as a box-to-box player and I am trying to score a goal."
