That article I think highlights what the issue is for Jost. The dude sees himself as a star player who simply hasn't broken out yet. He thinks he's a great player. I don't think it's a coincidence that he just had arguably his best regular season game ever when he was put on the 2nd line and the top powerplay unit. The other factor is he just got a boost from not being traded, which must mean Sakic believes in him. Mentally, he's very confident right now for these reasons, and we're seeing him play to the best of his abilities.
The problem is that he hasn't shown any evidence whatsoever that he can sustain this level of play when he's inevitably dropped down the lineup and not given key powerplay time. He just gets down on himself and thinks he's not living up to his potential.
What he needs to do, in my opinion, is learn to accept that he's going to be a bottom 6 player in the NHL. He's got to get satisfaction out of games where he doesn't score, but plays good defensively and has good shifts. He's also got to continue working on his speed, because he's never going to be big enough to out-muscle NHL defensemen.
Mainly though, it's mental with him. Remember: he's the youngest forward on the team outside of Kaut, and he's the same age as Makar and Girard. We can't expect every 21 year old to be mentally tough and mature. At the end of the day, it will be up to him if he's an NHL player. If I'm right, his best avenue to success is to start emulating the Nietos, Calverts and Bellemares of the world rather than thinking he belongs with MacKinnon and co. Not every player can make that mental transition. But if he does, I think he could be successful. And maybe, counter-intuitively, by learning to be a bottom 6 guy he'll play well enough to earn a top 6 role as a complimentary guy.