Mentality is not about X's and O's. It's about how you are approaching things. The hardest rebuilding job in college football is taking over a program that has accepted a losing culture. In some cases it can take many coaching cycles to fix that issue. It hasn't been fixed at TN going on 20 years. Nebraska is trying to fix it too. Oklahoma once had the same issue. Alabama had some down time but not severe - they never became broken.
You can coach up a player to perfection for how to block a certain play, how to run a certain route against a specific look, and all of the other stuff coaches do, in the end mentality can be a struggle for coaches AND players and right now it's a battle to get over that hurdle for the staff and players.
They pushed hard to play football this year. That was a great sign of a changing culture. Then they went into the Ohio State game playing hard and pushed Ohio State for almost a full half. Then pushed for a non-conference game after the WI game got cancelled. All signs to that point were looking good.
Then the Northwestern game came around and the offense came out strong, defense played good, got the lead. Then things started to hit a wall and it was regression from there. Then Luke came in and got a spark. Nebraska put up 420 yards against Northwestern, an entire football field plus more yards against Northwestern than any other team Northwestern has played this season.
The team then came out and played hard against PSU and won. Offense struggled.
So now you are 2 quarters against Ohio State, most of the game against Northwestern (redzone play that kept that a 14+ Nebraska win), struggles in the second half against PSu on offense, and then IL they looked lost, flat, didn't executive much of anything well.
The trust of doing your own assignments and those around you to succeed are all about the development of the right mentality that wins games. It sounds easy on paper, but these are fully grown men that want to get paid to play the sport at the NFL level and when you don't do the right things mentally throughout the entire week of preparation you have already lost before kickoff.
People will go and say so and so sucks, fire everyone, it's all 100% wrong. It's really about the players learning how to win back-to-back games, how to lead a program during the week of practices, and the coaches being able to read things and know what needs to be done to fix issues that come up. Like I said earlier, it sound easy, but it isn't.
There is only one way you solve a losing culture - winning. Once Nebraska wins two games in a row the desire to win 3 in a row becomes stronger and it keeps growing.
There will be a breakthrough period that will come, it's just a matter of when.
And, yes, developing a deep passing game that is needed in the offense is part of the mental side of things - trusting the WR, WR trusting the QB, reading a defense correctly, adjustments on the fly, and so much more.
There will come a day when Nebraska's offense is not just putting up 500 yards in a game and scoring 21 points, it will become 600 yards and 35 points