Theres many people out there that are still highly functional still have a good social life and act like there's nothing wrong but are fighting with depressive mental states.

Just because you seem like you have your life together doesn't mean you can't suffer from depressive emotions and thoughts.
Which means you could have mild depression.

I've been optimistic for years about my mental state " that everything is fine approach" its denial. when you are feeling something is wrong inside your head then theres a great chance there is.

But I realised that I could have a thousand times better perception and healthier mind than I had, took me a while though.

You might not be clinically depressed or depressed enough to be on medication.

But you can still be slightly depressed. Where do you draw the line? Not even therapist know exactly. Because it's just way too complicated.

There could be so many factors coming into play with why you are feeling the way you are, but i'de put my finger on diet, since your external world seems to not be causing the problem, Maybe you are not consuming enough foods that promote healthy chemical production...maybe experiment with natural supplements? Supplements helped me but I had to find which one worked the best. I jumped onto dopa mocha, which helps with dopamine deficit, but there's loads of stuff out there that could help you.