Don't be too hard on yourself. Before I got my degree I was also a hobbyist and not particularly good at finishing stuff, and worried about all the job offers asking for a portfolio. But look, CA is looking for 3 years of game dev experience, which is much less than what it took him to get Stardew Valley done. This is not a case of "being better": you're only programming in your spare time, of course you're going to jump between ideas and projects! Frankly, the offer seems to target a very small core of experienced programmers in the games industry.
Heck, I'm a programmer in said industry now, I'm pretty confident with C# and my abilities but I can't claim 3+ years of game dev so I won't apply - even though it'd be the dream!
Hey, if you feel confident in your abilities, but don't have 3+ years, I encourage you to apply anyway. Ultimately, I just want to find people who are right for the job.