How can I be pro at string/json manipulations/regex etc?

I really suck at it and it really impacts my performance.

If you want to be a regex pro the first thing you need is severe brain damage

Just practice, it comes with time.
I learned it by doing a lot of content scripting (migrations, integrations, etc...)

What helped me a lot in getting more comfortable with regexes are sites like regexr and regex101. They got a huge text box where you can throw all kinds of strings into to test against, and a field to enter your regex, with helpful highlighting, syntax check, explanations, and inline help.

You can just fiddle until your regex does what you want and then get an explanation of what exactly happens.

Ridiculously helpful.