You are a masterful puzzle maker. This game is almost quite literally perfect.
I had nearly beaten the game before discovering the player's actual hitbox, which made many levels really difficult the first time around and much more creative the second and third times through. (It's satisfying to stand on a platform while multiple killer blocks graze your sides leaving you unharmed as you examine the level to ponder your next move.)
I'm not sure what else to say. I've beaten the game a few times over by now (admittedly, because I couldn't figure out the secret ending during the second playthrough). There are several satisfying aspects to playthroughs...
1) How memorable most of the levels are! The majority of levels are keyed into my brain and feel familiar, almost home-like.
2) How my latest playthrough probably took me 1/2 the time of my second, and that took 1/2 the time of the first. I spent a good portion of a day trying to beat the final boss first time around, and I just did it here tonight while keeping to my 1-hour time limit for beating the section of the game... The improvements based on learning in this game are so severe that I now consider what were previously the most difficult levels for me the most "fun".
I was disappointed by the original ending. Lacking a key or two there did not make me feel very good. The secret ending is a bit more satisfying and plays well into the theme. I almost feel as though it's a conversation the main character is having with themselves, to reassure themselves. A little odd for me, considering I seemed to dive into the depths of hell for the purpose of defying death itself... but whatever.
And I still don't know who the damned puppet master thing was supposed to be. My emotional support friend, I guess. Oh, well... someone's gotta be the third wheel on this wheelbarrow.