You know that feeling you get when you overlook something really obvious, not just once, but many times? And that lack of that particular knowledge causes you a great deal of frustration? And then, later, when you finally notice the thing that was under your nose the whole time, you suddenly become of all the unnecessary trouble you could have spared yourself if you'd only been paying proper attention?
The way to the end of the game was behind a door marked "exit." And while I could, as a defense, cite the other doors marked "exit" that eventually proved to be full of shit, this one was different. Unlike those others, this door was in the middle of a "real" chamber, and not directly taunting me by overlooking the hub area.
I keep going over it in my mind - how could I have missed it? You pretty much have to walk straight into it. The only thing I can think of is that the first time I saw it, the area right in front of the door dropped me down a pit. The second time I saw it, I skirted the edge to get right up against the barrier in front of it. My gun could do nothing to the (at the time) unusual red blocks (as it turns out, because it was behind a gun-disrupting field, another mechanic I had not previously encountered), and thus I assumed that the only reason this so-called "exit" existed was to lure me into the pit. On subsequent encounters, I ignored it entirely. Hours later, when I had the red gun, and thus everything I needed to get through, I had forgotten all about it.
And thus, I spent about five hours poking into various dead-ends, loops, and bonus areas, all the while thinking that I was advancing towards the end of the game, and then being repeatedly disappointed when my brain-wracking had little to no payoff.
It was only chance that saved me, in the end. I had gotten past the "wtf?!" gap that had previously stumped me, then past the room beyond, and into an easter-egg room that appeared to contain screenshots from previous builds of the game (or were they other games made by the same company - I'm not sure because, with one exception, they never bother to explain any of this stuff with text). I was certain that was the path to the end of the game, and thus I had to jump back to the hub and cool off for a minute or two to recover from my frustration.
All that was left was three chambers I'd already passed through. These chambers, I had dismissed as possible candidates because they all had an obvious puzzle that I had previously solved to unlock a new chamber. Thus, the open paths on the map had to be super-secret passages leading to dead ends. Why, one of them was from the very beginning of the game (I don't remember exactly, but I think it was the second or third puzzle I'd encountered), so there was no way that could be the way out.
Yet the process of elimination demanded that it be at least one of the three, so I went back. All the way to this very early and not especially complicated puzzle I hadn't thought about in hours. I crossed the gap. I went around the corner. I teleported back. I looked into the gap. I could see no cunning mechanism that would allow for my advancement.
Then, I did something that is going to seem very obvious, but which, I assure you, felt at the time like a last ditch desperation effort - I looked behind me. You see, it was a kind of muscle memory. I knew without thinking about it, that there was nothing worthwhile behind me. It was a dead end. There was no reason to look back because there was nothing back there I could get past.
And that was true, the last half-dozen times I'd visited the room, because I had not yet acquired the red gun, and thus could not get past the barrier that separated me from the exit . . .
Anyway, my plan for the last five hours is to aim for 100% completion. I've already got all of the available signs, thanks to my misguided attempts to explore the edges of the map. All that's left are two secret passages (possibly with puzzle chambers behind them, though I doubt it, because there are no more signs left to find) and the pink cubes.
I have not yet used a guide, because it seemed kind of pointless to me to speed up my playtime when I was aiming to go for 20 hours anyways, but if I get more than a little stuck with these chores, I'm going to go ahead and use one now. Thus, I'm not expecting it to take more than an hour or two to tie up these loose ends. After that, I'll try for a speed run (can I redeem my own obliviousness - probably not).