The striker signed for QPR in January 2016, saying at the time that working with former Chelsea striker Hasselbaink was a "massive pull for me".
Yet he returned only two goals in 31 appearances under Hasselbaink, who often deployed Washington on his own up front or on the wing.
Hasselbaink was replaced by Ian Holloway in November and both Washington and QPR have since found their form, with the 24-year-old netting four times in his past 10 appearances and the Hoops winning five or their past seven in the Championship.
Washington has therefore arrived on international duty with Northern Ireland this week with a spring in his step again, having admitted he was left with a post-Euro 2016 hangover working with Hasselbaink.
"Within the year I've gone from the highest point to the lowest point back to where I am now," said Washington, who made his international debut exactly a year ago on Friday.
"It's so important to have that confidence and it's mad how quickly things can change. Things like that, it's strange how much of an effect it had on me.
"It was a really tough time after the Euros to be honest. Going from playing and scoring and being the main man at Peterborough and coming to QPR and having a few disagreements with the manager, not really understanding how he wanted me to play, his style of play, it was really tough. It was an even bigger comedown coming off the summer.
"It was not so much disagreements as miscommunication, maybe, probably from both of our parts.
"I didn't really understand the job he wanted me to do within the team and I've always found playing up top in my own in the system we were playing doesn't really suit me particularly well. There was a lot of contempt on my part in that."
All that is in the past now that Holloway has arrived, even if a younger QPR squad has left Washington playing with the seniors when training consists of older players versus younger players.
One of the first things Holloway told Washington, a former non-league player who only turned professional in October 2012, was that he would have signed him had he been in Hasselbaink's shoes.
Holloway then had his staff put together a compilation of Washington's goals for club and country to restore the confidence he displayed during goal-laden spells with Newport and Peterborough.
"It wasn't that long... like 10 minutes of video, with the celebrations as well," Washington said, ahead of Northern Ireland's World Cup qualifier against Norway on Sunday.
"It was one of the first things he said to me - 'Nothing's changed, you've just lost that little bit of confidence'.
"I totally agreed with him. Little things like that have really helped me. He got the video guys to put together something and it's nice to look back and see what you've done and achieved.
"It wasn't so much watching those goals, it was what those goals meant, especially the ones against Slovenia and Belarus, they were really important in terms of the bigger picture and getting into the (Euros) squad.
"When I was on a good run at Peterborough, it meant I was going to get the move to QPR. It was really important and reminded me, 'Okay, I am good enough to play at this level'."
