This stuff is complex. There are no magic potions or hacks that work for everyone. No amount of positive thinking can fix physiological issues that you have because of some underlying condition. But it's quite possible to create new issues as a side effect of being depressed, stressed, chronically exhausted, etc. So, working on those issues is probably smart.

And each of those things can be caused by physiological issues as well. You might feel stressed because you don't sleep well. You might sleep poorly because you suffer from e.g. sleep Apnea. Which in turn might be because of a mix of physiological and other reasons (diet, weight, alcohol abuse, etc.).

Or you might be working too hard, which makes you stressed and causes you to lose a lot of sleep. Different causes that have similar results. Including long term physiological results. Your brain can actually get damaged if you chronically abuse it or neglect it. Many "between the ears" type problems are actually physiological.

Root causing your issues enables you to deal with them properly instead of fighting the symptoms.

Anyway, I take vitamin D and a few other things. Getting yourself checked out regularly once you hit middle age is a good idea. There's a lot of stuff that is long term lethal that a checkup can detect early. And some of it is fixable. I have the usual cardio vascular challenges that many people struggle with because of a combination of genetics, age, and life style. And indeed a vitamin D deficit.

I was also recommended to consume more omega-3 as well. Eat salmon. Work some flax/chia seeds in your breakfast. I put flax seeds in my yogurt and use it as a thickener in sauces as well. You have to grind it to dust for it to get absorbed properly. Dirt cheap and it doesn't mess with flavor/texture too much. I keep a jar of ground flax seeds in my fridge. Takes 2 minutes to top it up every 1-2 weeks or so with some freshly ground seeds.

But I'm also aware that me being a stressed startup founder has health consequences that a few pills and suplements won't fix for me. I need to actively make sure I get my rest and sleep. I deal a lot better with stressful situations when I'm well rested. And I seem to be better at avoiding getting in to those as well. And I feel happier. Sometimes the best thing I can do for my company is having a proper weekend or going to bed early enough that I can get my 8 hours of sleep. You can survive on 4 hours (been there done that), for a while. But most people are not at their peak performance if they do that. And it's not good for you to work yourself to exhaustion all the time.