Speaking only for myself, in terms of mental endurance I believe some of it comes from character, and some from things I went through. I'm the sort of person who draws a long-term plan to achieve my goal, and when something goes wrong I try to adapt to new circumstances. I also don't like breaking promises, and I did promise to finish the game -- that's more of an anchor for me than most people imagine.
I also believe that writing a novel (visual or otherwise) just inherently demands a kind of resistance against discomfort. The process of writing is often fun, and it's always enriching, but when you set out to do it for years or decades you realize that on the long term it will almost always demand that you cope with a high amount of mental effort. And, past a certain point, when this becomes one's job, that effort becomes physical as well.
I'll also admit that a bit of an obsessive trait is probably positive, too. The slight obsessive capacity to latch on to an idea for years until you see it to its end.
Now, in terms of lived experiences... I think a key thing for me is that I started publishing my writing very early and, crucially, I ate a lot of shit. I know what it's like to spend years writing a book that will not move more than 300 copies, and what it's like to put your work through the Amazon Kindle algorithm hell hole. That really set my expectations for how bad things could be.
So, when I came over to FVNs... Well, some people might think of all the issues in this space, but my attention went to how much better this is than traditional publishing. It's easy to say that things are great now, but even back during our first release, when our number were far from impressive, I quickly realized how good this readership could be.
Friendship is important, too. I think it's a lot harder to give up on a project when you're accountable to others.
All of that said, if I had to say what was the biggest factor for me... I simply love writing. I love the act of writing, I love the outlining, I love chasing ideas. I don't like programming -- that's evil and heteronormative -- but I love almost everything else. And when you love something it's not really that big of a burden, is it? Even if it makes you tired at the end of the day.
Now, in terms of logistics, right out of the gate I think it's important for me to bring up that I live in a cheap country and was able to get some decent savings going. I say this as a segue to the bare reality that, you know, you can't make creative work if you can't somehow pay the bills. And, in our case, I had to put some of my savings into the game to get things rolling. This wasn't necessary when Minotaur Hotel was a project among friends, but we had to make the transition into becoming a game development studio that pays its team, and with that comes a number of realities that any small business owner will have to deal with.
Now, during most of the project's lifetime our team worked under a consensus-based decision-making format. We discussed the plans for the game and, almost without exception, we were capable of reaching an agreement between the ENTIRE team. I tried to make things as horizontal as possible, which has its strengths and weaknesses, but over time the team saw me as the director nonetheless. I think that format was almost always good and an important factor to our longevity -- everyone in the team was able to bring up issues, everyone's input mattered, but at the end of the day I made sure things were thematically coherent always. This keeps the project together and going.
Now things are a little more professionalized -- we have to worry about deadlines, financial realities, scheduling, etc. Things had to become more vertical and organized. It changed, but even now we carry some leftovers from this format (Awoo is director over Wolf's writing, Nemo directs Kota, etc).
There's no way around it, I think longevity here comes down a lot to how the team manages itself. Leadership matters.
Does this answer your question? If you want me to dig some more in regards to any of those points, just say so.