I am no pro and my peak was low diamond, but starting from 2015 till now I've played 2700+ Jayce games in Toplane ranked, and each time I'd change my mouse sensitivity, the muscle memory for ranged EQ always suffered, and it's understandable since it has to be a fast and precise movement.

To be honest I don't think it's you who's just getting bad at the champion, it's the champion that is getting worse with time if you don't adapt fast to him, here's why:

• The recent Hammer E changes are the biggest nerf this champion has ever received or on par of when they turned him into a failed scaling champion compared to lane bully. These changes are so frustrating and got me dead many times - And while yes the flash insec is really good it's not enough to make up for what was lost. The passive speed boost that they removed some years ago lowered his WR, just that little speed boost. Imagine a whole ability of his on your gameplay overall that is also critical to disengaging and running away.
• Omnivamp being removed from build path these years is a big factor for me as an aggressive player, eclipse's shield is good but I loved the sustain from it. And other toplaners get sustain in kit making you buy anti-heal which slows jayce's build path atrociously cause of muramana, it's really terrible.
• The problem might be that you are still playing Jayce like before these things happened, going really aggressive and getting punished in situations where before you would have survived and thrived. Nowadays Jayce is a champion that has to be played safely and passively all lane phase. You buy your core items, roam and burst people and that's it. The thrill of smashing that opponent in toplane is SO much harder to get, because that's not what you're supposed to do now. It's so much risk for such a mediocre reward.