How do you stop doubts and worries?

If this question is in the context of building a product, you don't. You ignore the doubts and worries and ship as fast as you can, as much as you can.

I've found what works really well for me is my overthinking/worry/uncertainty comes from a lacking of knowledge about that specific topic. So each time I push myself to acquire as much knowledge on it as possible and that helps reduce my doubts.

Anytime a doubt or worry comes to mind, try to substitute that thought with an accomplishment or progress you’ve made.

I advise you to do more of what you like and sometimes not to bother with anything.

This is easy for me personally. A couple of good friends died in accidents a few years ago. They were much younger than me. When I start worrying about things or getting stressed I think of them and think “hey, I’m still alive and well”. It’s just not worth it worrying about work or money. I’m confident in my own abilities and I’m sure not lazy so I guess I’ll be alright. I also try to live way more in the present and not dwell on what might happen in the future. Playing with my children, spending time with my wife are all great things I love to have a break from work. When I’m in work mode I love it and am fully dedicated but I try my best to stop worrying about things that are out of my control, life is just way too short!

Stopping doubts and worries is often about learning how to manage them effectively, since completely eliminating them is impossible and sometimes even unhelpful.