This is just my opinion, but when I was in OP's shoes I found this to be the least helpful piece of advice I ever got. I was lucky enough to realize that programming stressed me out early on, early enough that I could switch majors into something that I liked better. But I still wanted to be a programmer, even if it was just as a hobby.
What I didn't like about being a real career programmer was the time stress, the expectation that you'll write code that works, and especially having to write code for someone else, where you're trying to translate their ideas and trying to do so correctly with limited information.
Contributing to open source sometimes (in my experience) embodies all of these in one. I've found a lot of really helpful open source projects where this isn't the case and everyone is nice and helpful, but you'll also find some times where the maintainer is mean, doesn't like your code, your code doesn't fit their style, and they're going to make sure you understand that. And if you spend your time writing good code, then do a pull request just to find out someone had already beaten you to it? Oh god... I'm stressing just writing this out.
Even if it's just 5% of the projects that are like this, to someone who already has crippling anxiety about their skills as a programmer, being put under public scrutiny is unthinkable. One bad experience and you might be gone forever.
What actually helped me was getting a job in another field and finding ways to write bash scripts or python scripts to help, with each script increasing in complexity until I had hundreds of small programs that I designed and wrote and improved over time. Zero stress, zero expectation, but when they work it feels amazing and is a great boost in confidence.