So much good advice here already, so I'm just going to send solidarity to you in dealing with this, it's so taxing and pretty scary.

Last summer I did a cold shower at the end of the day just to be able to sleep. We are lucky enough to also be able to go stand in a lake until the core temp drops. Many here are advicing against this, but my middle aged perimeno body really did better with sleep after standing in a cool shower long enough for my core temp to drop to where the shower no longer felt cool. Ended up with a terrible water bill and a bad conscience with the water comsumption, but sleep is important and data centers use a billion showers worth of water for no good reason.

I also like to eat cool things, like watermelon slices frozen or cold soups.

I read somewhere that there is a way to diy paint windowds with white chalk for effective cooling, but the various coverings in this thread are probably more accessible.

I also read a tip somewhere on just putting a cool water bottle or something in your crotch, this will cool you down very well apparently (if you can tolerate it).

Edit. Adding that with the shower also never dry yourself, but just let evaporation do the work, it cools you so well. I like to go stand in front of the fan naked if it's very hot, it's pretty nice and helps you sleep. Sleep fully naked and with no insulation on you, I do a cotton bedsheet only because I can't sleep fully exposed for some reason. I've also gone to sleep on the floor, because it's lower and all the cushioning in a bed gets very hot when it's already hot.

The problem with the Soviet block building I live in is that the thick conrete walls start to accumulate heat after a few hot days and it's impossible to get anything to cool after a point. None of these have AC.