With heat unfortunately I cannot do as much as with cold - I need moreless two weeks in the heat to acclimate correctly and then my performance is somewhere close to non-hot-weather performance (I'm a runner). But unfortunately at some moment this also stops working when I'm doing some very high effort - active cooling is required (aka splash yourself with water, ice etc). In some conditions (especially hot+humid) human built-in active cooling mechanism also doesn't work well - sweating is awesome, but sweat no longer evaporates effectively in humid conditions. I also say it's easier in cold - you could always put more clothes on you if need them, but in the heat? There is only as much clothes as you can take off.

I'd agree it's somehow mental discipline - I can also ignore pain, it's there, but well I don't have to act on it, it does not have to have any reaction. It used to be a cool party trick back when I was a teenager... Again, not a medical professional, if you experience pain, it's body trying to tell you something is wrong, most likely you should listen to it.