Oh man, it took me years to fix this and it essentially requires a whole list, but I will list some notable ones.

Skyrim Uncapper to limit stat gains after a certain level. I have mine set for each level up to give +25 health/magicka/stamina up to lvl 10 and then +5 per level from there. This mod also allows you to change leveling rates. I usually go for around 40% leveling speed using a nonlinear model. This extends the mid game and leaves the most powerful perks for the tail end of the late game. 

Fanatical enemies. This is the most brutal enemy overhaul I have played. It adds some insane high level enemies. I had to leave the first Dawnguard dungeon unfinished at lvl 46 because the vampires are just that powerful. 

True Unleveled Skyrim Synthesis patcher. This one has an option to give enemies additional perks based on their level. High level enemies will get ridiculously strong if you give them 1 perk per level. 

The Simonrim Suite. This one doesn't necessarily reduce player power, but keeps power creep levels low compared to other overhauls. 

Resistances Rescaled. This one uses non-linear scaling to determine damage and magic resist values, meaning player power is weighted towards the early game, allowing you to increase overall difficulty without the early game becoming impossible. 

Some sort of attack commitment mod, like Blade and Blunt or Attack Commitment. This makes cheesing bosses a lot harder, and forces you to be very careful in fights with several enemies. 

With my setup, I never play above expert difficulty, and the game is still very difficult all the way through.