A "thing" is just such a vague and general concept that is useless. If either your job or your relationships can be reduced to only mastering 1 thing I really envy you. If you say I am being unfair and of course it involves many sub-tasks , so why dont just choose to be good at life? So you can concentrate in just one thing and still have 2 other "things" to spare.

Now, less snarky and more serious, in any activity or discipline you choose, there are people who are dedicating their whole life to it, so you have 3 options.1) Hope your natural talent will compensate the less hours you dedicate to it (this is almost always false). 2) Accept you will never be as good as the crazies who spend all of their time on it.3)Join the fray and invest all your time on it.

Almost all great creations in humanity, things like Einstein's theory of relativity, Dante's Divine Comedy, Egypt's pyramids are the result of incredible and exclusive dedication for many many years.

What you say is absolutely correct ;-) However, I still find it useful to think in these terms. For example, I'm working on a game in my spare time and streaming (most of it) on Twitch. But I'm also making cheese and doing a lot of research for it. I keep thinking of writing a book about cheesemaking because the information available to normal people is by and large pretty awful. So I think to myself, "Are you willing to give up your game in order to write your book?" And the answer is, no. I may slowly write a few things here or there, though (just to reduce the amount of time I spend answering questions on reddit!) But I think a book would be a very bad idea for me even though it is sooo tempting.

Just blog. It's like a book, except you can stop whenever you want and you haven't lost anything. (That's what Eliezer Yudkowsky did, and those blog posts eventually became a book.)

Didn't Einstein had a day job at a patent office while developing the Theory of Relativity?

It's worth noting that much of his work at the patent office related to questions about transmission of electric signals and electrical–mechanical synchronization of time, two technical problems that show up conspicuously in the thought experiments that eventually led Einstein to his radical conclusions about the nature of light and the fundamental connection between space and time.

The special theory (among other goodies), but I am willing to bet nobody was investing more quality time at that problem at that year(Theoretical physics was a game played in a few European cities). For the General Theory he spent 10 years (1905-1915), he worked at the patent office until 1909, and even yet, he got almost scooped by Hilbert.

I think you also get better at some things as you get older, an some things require more upkeep than others.

For example, it's only been fairly recent since I dared to claim I'm pretty good at programming. And not even all aspects of it, but there are some aspects where I really notice I'm better at it than my co-workers. I've been doing this professionally for 20 years, so I guess it's about time I got good at it.

I've always thought I was pretty good at sailing; I think I have a talent for it. Used to do it a lot back in the day, but haven't been able to sail much in recent years. I'm not doing the upkeep, but I don't think I forgot any of it. I think I can jump in a boat and sail safely while teaching someone else. I don't think sailing costs nearly as much upkeep as something like physical fitness, which drops pretty quickly once you stop exercising.

I'm good at cycling, but doing the upkeep for this is easy: just take the bike when you go anywhere. I'm not as fast as when I was younger, but still faster than most.

I've been playing RPGs all my life, but I think I only recently got good at GMing (I've done it plenty of times before, but it was very hit and miss, and the misses sucked bad). But that's something I've been putting a lot of time and effort in lately. I clearly don't have any natural talent for it, but I've managed to accumulate quite a bit of experience over the decades. I still lack the discipline to do thorough prep, but experience fills in a lot of the details.

Despite having played a ton of different instruments (drums, keyboard, guitar), I've never gotten good at music. I lacked the discipline to practice every day, and though I would like to be better at it, I'm clearly not as passionate about it as I am about gaming.

I don’t quite agree with this. It depends on how often you change your mind about the third field, and what your standard for “quite good” is. If you put a few years of effort into a field and get to a high level of competence and productivity, that skill may stick with you for a long time, even if you fall out of practice.

For example, let’s say that you play piano, and you push yourself to a new level of skill and learn a few more advanced pieces. The decay rate for that skill is quite long, and with a very minimal level of work you can keep those pieces fresh for years or decades.

I concur; I was AFK for a while (pursuing other hobbies, living abroad with no access to a full-sized keyboard) but picked it up, and improved beyond my prior capabilities, pretty quickly.

I've noticed trouble playing advanced pieces within a few days to a week of little to no practice. But I guess everyone's different.

Very much in line with my experience, but in fact for me it wasn't even 'being quite good at,' but just sufficiently managing and exceeding minimum expectations. In my college years that was, for example, studying, working, and managing the faculty magazine. I had no time to give to friends or socialise even. Relationships are definitely taking up one of these 'slots,' so to say.