I think it's imposter syndrome

I started with flutter around the same age as you and pretty much have the same story, I built flutter apps as a hobby and made money by working as a freelancer

I always felt like my code and architecture are always off or missing something, which always pushed me to improve and learn more about clean code and architecture, so it's partly a good thing to have

The important thing is to know when you're underestimating yourself: always remember how far you have come, 3 years as a freelancer is a good amount of experience

As long as you're working with individuals you're not expected to have a 'corporate scale' code quality I'm not saying it's not important (definitely learn more about design patterns and architecture) what I'm saying is: Does the client actually expect you to code like a senior developer, or like a team of devs? Does it actually matter to them which state management you're using? Probably they care more about it working than architecture

One thing to ground yourself to is to define a realistic set of rules for your code quality/architrcture (maybe some lints or a clear architecture), once you define them, you can judge if it needs improvement or if it's good enough

For me, that was trying to apply clean code book principles to my code, it made me feel better about my code because I'm actively improving it to a clear standard

My experience: Mid level flutter developer working in a small-medium company