The Englishman has had a successful career that includes spells at Albion, Villa and Walsall but he has struggled to make his mark at Wolves since arriving for £10million last summer.
Johnstone made just 10 appearances in all competitions and played understudy to Jose Sa when Vitor Pereira took over as head coach.
As he discusses the mental side of the game, Johnstone says his experience has helped him deal with the difficult moments in his career.
“When I was younger, I probably struggled to a certain extent with setbacks, but as you get older and the more games you play and the more experience you get, it’s just one of those things that you can deal with," Johnstone said.
"When you have a setback or make a mistake, it’s obviously not something that you’ve done on purpose, they just happen. If it’s during a game, I just try to wipe it and focus on the next thing, or if it’s something that keeps occurring, I’ll work on it in training.
“I’m just that type of person anyway who, good or bad, I work hard and don’t get too high or too low.
"When things are good, things are obviously easier, more fun, and when you have to get your head down and work, you have to do that without getting frustrated. You can’t get too high when the times are good because in football, times change very quick and it can soon turn very low, so I just try to not go to the extremes of either.”
However, the 32-year-old admits he finds it difficult to move on after a game and relishes the change to focus on the next fixture.
He added: “I wish I could switch off straight after a game, but definitely after the game and then into the Sunday, I’m thinking about the game, good or bad.
"My mind probably doesn’t come off it until you start working towards the next match on the Monday or the Tuesday.
“As I’ve got more experienced throughout my career, I’ve found some things easier to cope with, and other things more difficult. Like we said before, things change quickly in football and you can’t take things for granted just because you’ve got more experience.
"That’s the way I look at things, and I’m always looking to improve every aspect with every game that follows. Of course, experience has helped, because when you first go into League One and then the Championship, it’s new, but you get used to it, then the Premier League is new but you get used to it, so experience does help, but I try not to let that control me.
"I still want to feel like it’s new, have adrenaline and get excited for the game.”