MSU didn't know how to play a game when they had such a large lead. If it had been a 7 point game, they'd have kept doing what got them there. However, Texas had to radically alter their mindset. MSU didn't have to, but did.
It's fun to bash Arch, but the fact is, he's a crazy good QB who's standing behind an OL that can barely sort out a direct rush. If the defense adds extra blitzers, they got no shot. Someone's running free to the QB. Very few QBs can play under those circumstances (as he gains experience, he and his receivers will get on the same page regarding hot routes, but that's a different topic). When allowed to play upright, his knowledge and ability to deliver passes on target is what you'd expect from a top tier QB.
When Texas chucked the running game and extra TE in favor of a speed assault, MSU should have kept up the pressure. The offensive changes didn't make it harder to pressure the QB - it made it easier. Getting to Arch would have bogged the Texas offense down (or caused turnovers). Instead, MSU tried to layer their pass defense into some creature they weren't familiar with. They voluntarily stopped attacking the QB with extra guys.
The result was evident. Arch knows the game and will find his guys if you don't bother him. Also, never punt to Ryan Niblett if the outcome of the game is in doubt.
There's more to the mental part of the game than most fans want to admit. Texas was trying to find a way to win the game. MSU was wondering how they were going to lose it. I told my daughter that somewhere there was almost certainly a State fan that loves his Bulldogs as much as I love Texas, and he's trying to tear his heart out right now.
Look. Four games away from home, three true away games, with a 3-1 record in the SEC is a tough ask for anyone. I see how Texas is playing. It's frustrating, but I hope they're learning something each week. Learning with a W is better than learning with an L.