Hey all. I kinda posted this back on Reddit a bit ago, and even made a written review about how much I liked the game (https://www.gamersdecide.com/pc-game-reviews/celeste-review-read-you-buy if you want to read it) but I really want to see the discussion blossom a bit more, so a lot of what I'm writing here is a copy/paste, but I think it's an interesting discussion nevertheless.
As someone with a history of negativity and feeling I need to "better myself" the themes of the game become super clear early on. Madeline wanting to climb the mountain of Celeste is less so of adventure, as one would expect in a game like this, but conquest. Conquest, in this case, is about conquering one's issues - the "conquest" of happiness/wellbeing/peace of mind - and the mountain represents both a physical "pillar" of this but also a subtler one I'd like to mention later.
Early on it's quite clear Madeline is aiming to climb the mountain to "get" something out of it, and of course, for people who played the game, it's her attempt at escaping her shadow self, the parts of her she may dislike, the elements of herself she wishes to escape. This a surprising take on the theme, as even games like Persona have this "darker side" be a type of hidden side, but Part of Me is simultaneously the "antagonist" and the obstacle to overcome.
What makes so much of this very interesting is we're often taught or just generally believe that our goal is to conquer over obstacles, to "defeat" the enemy, even if it's part of ourselves. The saying "I want to get away from myself" is quite true in the lives of many people myself included. But to quote a title of a book by Jon Kabat-Zinn, wherever you go, there you are. Whether you're trying to go elsewhere, or like me and live at a Zen monastery for a time, or Madeline trying to climb Celeste, going elsewhere, "out there", is never truly going to work.
The metaphors for this external conquest were rather apparent to me. Consider the scene where Madeline falls down the mountain in Chapter 6. Madeline's upward efforts - her "conquest" to escape the dark side of herself - push her downwards, down the mountain. Her efforts to escape herself only leave her further downtrodden. Very much like happiness itself, by it being defined as something out there to get, the very act of chasing it makes it harder to even see in your life, and in a similar metaphoric sense, Madeline's efforts to ascend her negative side only force her to descend because of it. Part of the reason she ends up climbing back, both in a personal sense and in a sense of climbing Celeste, is when she comes to terms with the fact she cannot escape her own shadow, that she must accept herself completely, and it's in this sense that she not only overcomes her obstacle, but is able to finally and totally climb Celeste.
The mountain of Celeste isn't just a metaphorical obstacle for Madeline, but a metaphor for acceptance and moments of realization. If many of us have objectified something on a mountain - let's say, overcoming an internal strife - as a goal or target to get, the mountain is seen as something to climb. But instead, the mountain is irrelevant, for the realization one can have can happen at any place of state regardless of where you are in relation to the mountain. The mountain, in this sense, was never an obstacle; the mountain was always internal, always inside. The mistake many of us make, and the mistake Madeline also makes is thinking it's "out there" to be overcame, and in persiting in that direction we experience follies and fallacies. But it's once we flip the script, shall we say, that we realize the obstacle in much of our lives is us, our images, our wanting X and to get away from Y, we see any problem within ourselves can often be confronted wherever we are, and we don't need to climb mountains, or live in Zen temples in order to have the ability to do so in the present.
Do hope my ramblings here were of interest. As someone with a background in the study of mind and suffering, I got a tremendous deal out of the game, and I hope that shows in what I wrote here. I got much more out of this in one run than I'd expect if I speedran it. :P