Longstaff was left out after Newcastle United's defeat to Sheffield United at Bramall Lane in mid-January.
And the midfielder only returned to the starting XI earlier this month.
The 23-year-old has spoken about head coach Steve Bruce’s decision – and his reaction to being left out – in an interview with the matchday programme ahead of this afternoon’s home Premier League game against West Ham United.
"To not be involved for so long was really frustrating,” said Longstaff, who broke through at Newcastle the season before last when Rafa Benitez was manager.
“Obviously, I have my own views on it, and whether I feel it was right or wrong, (but) it doesn’t really matter. I don’t make the final decision on it. All you can do is keep yourself ready.
"I know that in the two months between starts, I trained really well – probably the best I've trained in the last two years, I would say.
"I know it could have gone one of two ways; either me sort of sack it off – which isn’t the type of person I am, the way I’ve been brought up, or how I do things – or focusing on myself, setting myself little targets every day, and ultimately training to the best of my ability, forcing them to give me a chance.
"If I did get that chance, then brilliant. I was going to try and take it.”
Longstaff believes he’s stronger mentally for his experiences over the past couple of years.
"I think now I’m a much strong person mentally, and I think I’ve probably learned to deal with the expectations that me and the people around me put on myself,” said Longstaff.
"I probably got caught up in a bit of trying to please everyone in a way, whether that was people’s expectations of us and me thinking ‘yeah, I do need to try this, or try that, in this situation’, where, in reality, you don’t.
"Last season was tough with injuries and stuff, but, this season, when I’ve played, I think I’ve gone into games and done what I’ve wanted to do.”