try to rememeber threshold and it will get easier. You don't need to remember whole range just bottom of opens/3betting etc

Also, dont feel like you need to perfect. Deviating few % in most spots is just whatever. No one knows for sure how much EVs are certain openings/defences/3bets having anyway due to rake structure/population adjustments.

or buy a subscription at preflopguru. they have some solved gto ranges and a nice trainer.