Patrick Bamford admits his career stalled at Chelsea, but sees a bright future ahead now he is back at Middlesbrough.
The striker, who spent the 2014-15 season on loan at the Riverside, has signed a four-and-a-half-year deal after Chelsea accepted an undisclosed offer.
Bamford enjoyed the most successful spell of his career to date at Boro, earning a place in the Championship Team of the Year.
The 23-year-old joined six different clubs on loan during his time at Stamford Bridge, most recently making six substitute appearances for Burnley in this season's Premier League.
"I'm delighted to be back," he told Sky Sports.
"It's something I pushed for when I knew I would be leaving Burnley.
"The last 18 months have been really frustrating. There were times when I was upset driving home from training. I was thinking 'I need to do something here' as I wasn't enjoying it.
"My development has stalled because I haven't played as many games, but I still learnt things in training. I've changed my approach to training, which is important. I've also toughened up mentally.
"It's been hard. When I was last here I was almost playing every game, then I went to Palace and didn't play. Then I went to Norwich and played a little before moving to Burnley where I barely played at all. It's been tough but I'm ready for this next challenge."
Bamford failed to make a single first-team appearance for the Chelsea senior side.
The striker leaves the club with a heavy heart but his desire to gain more first-team experience was a key factor in his decision to leave.
"My dream was to play for Chelsea - maybe I was bit naive," he said.
"I still have that dream. You never know what's going to happen in the future but for now, even though I was sad to leave, it was one of the happiest days of my life. I was delighted to be coming back here. Now I've got to help Boro push on and see how far we can get."
Middlesbrough boss Aitor Karanka always spoke positively about Bamford despite the striker returning to Chelsea after his loan spell, which saw him score 17 league goals as Boro lost out in the play-off final to Norwich.
The relationship between the two remains strong and Bamford is looking forward to working with the Spaniard again.
"He understands me as a player and as a person," he said.
"Foreign managers have a different outlook on everything. Everyone will say I'm quite relaxed and easy going. That suits the way the foreign mentality is. He's very good tactically - he doesn't leave a stone unturned. He reminds me of Jose Mourinho - I spent a pre-season under him and you can see that when he came here. He's a good manager and I look forward to working with him again."