This spring, I had a noticeable loss in control and velocity.
I started to think about my mechanics, trying to change in hopes of regaining my previous form.
This led to me, for pretty much the first time ever, actually thinking about how to throw whenever I stepped on the mound, and eventually whenever I held a ball.
It used to be just throw and the catcher wouldn’t even move his glove, for any pitch (fastball, curve, slider). It would do exactly what I wanted and end up where I wanted (usually not in the middle of the barrel)

Has anyone else had a problem overthinking and have any advice on how to just throw again?

of course there a million different answers that could lead you to success but heres the one tip i can give you. try it and come back to me with it.

go play catch with someone get in your wind up, whatever it might be, start throwing. after a couple pitches, try changing something in your throwing action whatever it might be, whether its taking a step back instead of sideway or hands over head before pitching or feet closer together before pitchig from the stretch even a different leg kick would do the job. this will lead to start focusing on something else instead of actually where the ball is going or how fast it is. you can throw a strike and do it effectively, you already did it times and times again so you dont need your head to mess with you.

Thank you, I’ll try that when I get a chance
I know similar has worked for me when diving (also a springboard diver)

One problem is that I unfortunately changed some things in my motion, so it doesn’t even feel natural anymore, not effortless.
Want to do something like a reset on throwing and start over
I don’t know how I can get back to that

ok try that then, start pitching with the no wind up delivery. you take a step back and then come straight forward, no leg kick or anything. it has worked for guys like jim katt later in his career and don larsen the guy who pitched a perfect game in the world series. really you get back to the essence of pitching, pure basics.of course yu probably wont stay with that but that will help you get back to a very basic pitching motion and so reset your technic to then work from there.

Take a good look at Don Larsen’s no-windup delivery—it sounds like this could be the solution to the problem you have with overthinking.
I’m reminded of the old story, or poem, or whatever, of the poor benighted centipede who had no trouble getting around until one curious frog asked him how he knew which foot to put down first—he started thinking about that and suddenly was no longer able to move. You seem to be in a similar situation, and it could well be that you need to get back to the basics, as you said. Look what it did for him; he pitched the only perfect game in the World Series, and that record still stands.