You're going about it the wrong way. You shouldn't compare yourself to a sim at all because Sims are not accurate representations of any fight in the game. Sims are essentially "If I can stand still in a fight for 5 minutes, just chain casting an absolutely perfect rotation into my target without ever stopping to use a defensive, moving to avoid an attack, interrupting a dangerous cast, switching to a different target or using AoE spells, this is the DPS I'll be able to do". That simply is not realistic at all, and the only boss of that type for all players in the raid we've had so far is Patchwerk back in Naxxramas. Sims are literally the perfect fight and unless you're EXTREMELY lucky and never need to do anything extra for any mechanic or special task and NEVER make any mistake compared to perfect play, you're NEVER EVER gonna reach that point.
There are better solutions for this. There is a tool called Wowanalyzer, which supports most specs in the game and analyzes your public WoW log parses and determines what you're doing wrong and how to correct it. Now, I can't guarantee that it is fully accurate for your class, but a lot of the advice it gives is generic and as such applies to most encounters, players and classes.
In addition, every WoW class has a dedicated Discord server (you can find these using Google, there are too many to link them all) where expert volunteers for every spec are willing to provide interested players with a personalized analysis of your log, including things Wowanalyzer can't provide suggestions for like positioning and specialized tactics for dealing with specific mechanics, like using unique utility abilities and talents to completely trivialize certain boss mechanics.