It’s like coming from gamepad to arcade sticks, when playing fighting games. There is nothing else you can do, other than train and play and git gud. Try the original DOOM, and I mean the first DOOM from the 90s. You don’t have to aim up or down, only left and right and its not precise as todays shooters. Maybe play that on a lower difficulty and see if you can get used to it.
Overwatch 2, which is Free To Play, has a training area and courses for heroes you can try. Take the simple Soldier, which is your average FPS character, and maybe you can get used to the controls. You don’t have to play online, just try it out against bots and do these training courses. Maybe that helps.
If you play such a shooter every day, DOOM, and others, than you will get used to it and build up muscle memory. Actually I find it exciting to learn new stuff like this and am a little bit jealous. ^^ Reminds me back when I came from console to PC and had to learn how to play shooters with mouse and keyboard.
Edit: Your age 30 is fine. Age is always an excuse, but mostly not true. I’m also from the 80s and grew up with 8-bit and 16-bit. Yet I learned how to play with arcade sticks and mouse and keyboard in addition to controllers. I’m 42 now (and proud of it). My biggest advice is, play every sort of game, not only you are comfortable with. And do it every day. git gud is the only way.