Steve McClaren believes he "compromised too much" during his time in charge at Newcastle United.
The former England manager took over at St James' Park in June 2015 but failed to make it through a full season at the helm.
With Newcastle languishing down in 19th place, McClaren was sacked in March. He had picked up just six wins in 28 Premier League games.
"I'm frustrated with my time at Newcastle and especially with the way it ended," McClaren told BBC Radio 5 Live.
"I went in with my eyes open and I knew the situation I was getting myself into, but the mistake I made was that I compromised too much.
"You have to go in and lead and manage. I compromised too much especially early on, instead of saying, 'we need this, we need this, we need this'.
"They've gone down because we conceded too many goals and didn't recruit centre-backs, and we didn't recruit enough strikers, we had injuries and bad luck and the main thing is the mentality.
"The mentality of the team was that they would play well against top teams, but they couldn't win the games they should have won against the lower teams."
Despite an upturn in fortunes following the arrival of Rafael Benitez, Newcastle were still relegated to the Championship.
Benitez has since agreed to stay on at the club, and the man he replaced insists the Spaniard must now be given full control of the club's transfer policy.
"They've decided to go with Rafa Benitez and I hope they've allowed him control of everything - most importantly he can control recruitment and maybe change the policy of the time which was only recruiting players under 25 and I think that restricted the recruitment process," he added.