..but when he has found the Path, at the moment of enlightenment, he understands that there is no reality and laughs at himself. This is why it is said: like in a dream."
~Nagarjuna - Mahaprajñaparamitopadesa - Chapter XI
When my mood is good, I can look on my experiences and feel good about them. I did the best I could, with the hand I was dealt. I have things to look forward to. When my mood is bad, those same experiences seem like a lot of misfortune, floundering, and futility. Those same things no longer seem worth looking forward to. In cognitive behavioral therapy, we're taught that we can choose which way we go. I've found that to be naive. Some things help improve the neurochemistry behind it, such as eating well, getting exercise - but these things work, because they alter the way the brain works, just as exercise alters the way muscles work. Ultimately, brain function is everything.
We all have good and bad experiences. Some are fortunate enough to have a life of more pleasant experiences, while others subsist in abject misery. In Buddhism, these are different realms of existence, and it's taught that we are best positioned to understand the truth, when we're somewhere in between, not at either end of the spectrum. Although not to dwell on it too much, as we are never well positioned to judge where we are on that spectrum exactly. It matters, but if you're positioned well enough to even think about it, that's a good sign, and it's best to move on.
In theory, state of mind is everything, for everyone, but in practice, circumstances can make that much more difficult to realize for some than others. Circumstances also matter.. but they are not the be all, end all, that the mind is. Deep in the machinations of neural pathways and and the chaotic interactions of biochemistry, our understanding of our environment forms, our circumstances, our world. What matters, and what doesn't.
Nothing matters, without the mind assigning its interpretations, comparisons, and values. This is why I say that mind is everything, and that I think the key to finding contentment in this world must lie somewhere therein. Not in what I accomplish, what I do, or what my past experiences have been. We need to do what we can, for the mind to be as healthy as it can be, as any effort we make can be undermined, if the brain isn't functioning as well as it could be. What we do in our external lives matters to that end.. but it is that end that we are most concerned with.
It is neurophysiology which governs everything else that we think matters. When we have mindblowing experiences, this is an alteration of neurochemistry. How that experience is derived is incidental. It's all about the neurophysiological changes that yield what we call experience, what we learn from it, what neurophysiological changes are ultimately achieved by it.
This must seem completely alien to those who spend their entire lives assuming what happens in the world around us is everything. In a sense, it is, and we are a part of that everything - but none of it means anything, without a human mind arbitrarily assigning meaning to it.