The hot face emoji means heat, sweating, warm weather, pressure, thirst, embarrassment, or feeling overwhelmed. It can be literal or emotional.
🥵
Literal use
People use it during hot weather, workouts, spicy food, fever, or any situation where they feel overheated.
Emotional use
It can also show embarrassment, stress, attraction, or pressure. The surrounding message tells which meaning is intended.
For a more dramatic "I am falling apart" feeling, the melting face emoji may fit better.