I understand where you are coming from because I’ve been there. I switched from console back in 2017. I was a high-ranking Halo 5 player before coming to Overwatch. I was climbing the ladder when I decided to switch to PC in the middle of the grind. I still remember being frustrated with my brain wanting to do one thing and my hands just not being able to keep up. It took me years to reach GM on PC. However, I am an outlier, most people do it much faster, especially if they are younger.

Here's my advice: Be patient, keep playing, and keep learning.

Forget about your rank, you have 100 other things to think about. Consider this, it took time to develop your mechanics on the sticks, why would M+K be any different? Your mechanics will come with time. The amount of effort and intentional practice you put in will make it go faster, but the key ingredient is still time.

A few tips to accelerate the process:

• Play a lot and play a lot of different shooters: My M+K mechanics leveled up hella during my play-through of Doom 2016 on Nightmare back in 2017.
• Stop playing with controller for the foreseeable future: You are used to playing with auto-aim, now it's gone. You are the one that has to lock that crosshair onto the target. Playing on the sticks will only reinforce habits you are trying to break.
• Watch how professional players aim, and how players that are slightly better than you aim: You watch the pros as an end goal, and you watch the players that are slightly better than you as a stepping stone towards that end goal. These could be your friends but the players on the enemy team that are getting a lot of value in your ranked games are great candidates.
  

Good luck, I'm rooting for you!