What do you do when VS Code refuses to "turn on" intellisense for a file?

I have this one .hpp file in a c++ project that's complicated. Too complicated. For this ONE fucking file, VS Code has decided to underline a bunch of random stuff in red and it refuses to show me autocomplete data. It doesn't show me member functions or variables and it doesn't show me functions that match what I've typed so far BUT ONLY IN THIS ONE FILE. The only reason I'm using VS Code at all is because this project is so mind-bogglingly complicated i simply can't remember everything and switching back and fourth between tabs, windows and monitors wastes so much time when you have to do it constantly.

VS Code always does this but you can usually get rid of the problem by closing it and opening it again. No such luck here. Having to do any work in this file without those autocomplete features is going to be such a drag. I actually tried moving the content of this stubborn hpp file to the h file but somehow VS Code is smart enough to troll me by underlining all the same stuff in red and not showing autocomplete data for the 1 block i copied in. How does it even know to troll me like this?

Other than switching to Windows 11 and using visual studio (like ms wants us to), what can I even do?

Also, there's nothing wrong with the code itself. g++ happily compiles and runs it and the relevant parts of the program work as expected.

Edit: hmm I changed the define keyword a little and its working for now although I wouldn't be surprised if that's not the case tomorrow. VS Code is good when it works but when it's being unreliable its just a glorified Windows XP notepad.exe with a dark theme.