Make sure your keyboard works before you play Hardcore
More detailed comment on what went wrong with my keyboard:
Basically, I'm using just this $20 Logitech wired keyboard and mouse combo from Aldi, and it was actually an improvement on my cheap second hand mechanical keyboard because at least it had better than 2-key rollover (seriously, I could not sprint and strafe diagonally with my mechanical keyboard, such a pain in the butt).
Recently, this keyboard started doing some odd things, where I'd go to run out of a cave, and the W key just wouldn't respond and so I'd turn and jump but wouldn't walk. No big deal I thought, only happened occasionally.
When I fell, I immediately put my thumb on the space bar and W, and for whatever reason, neither of them did anything and so I placed the water instinctively but because I was just sinking, I just got buried into my rather warm obsidian coated resting place.
And yes, I shouldn't have stood on the block, but I thought "I've fallen into lava so many times, I can easily survive it because I just place water and it's all good." and that's usually true. So I guess when it comes down to it, I was just extremely careless. Oh well.
Also, what mods was I using in that video? I made my own modpack up that's essentially vanilla but a bit more expanded upon (don't want to exactly say Vanilla Plus because I'm sure that name's already taken). Here's a screenshot of my MultiMC mods list (https://imgur.com/abPXK17.png) with the exception of FirstPersonModel not being listed there. They're all Fabric mods by the way, not Forge.