Thanks, that helped me figure it out. My issue was that during the climb after the Prometheus boss I was taking the right route instead of the left one you highlighted, so I was missing the temperature upgrade and the minimap. It's confusing that there's a path like that which lets you survive at first but actually makes the level unwinnable until you die and restart. I was also confused by having to go down into the scary damage area on Vangelos to collect the needed upgrade there, and especially that it required you to beat Prometheus first (so you had the temperature upgrade) to pick up the cold upgrade. I think I beat the game now (I got all three triforce shards but no clear ending, not sure if there's anything else left to do). Thanks for all the help, and for making the game! I really enjoyed it overall despite getting stuck.
edit: actually, I think there might be a real ending if I figure out how to get up through the green-walls room on Cornolia... I think I'm still missing one more upgrade, I'll keep looking
edit2: ok now I actually beat it for realsies. Wow, what a great game!