Thermostat is likely stuck closed. Buy new thermostat.
I suggest a Stant Superstat. 195 degree model.
Be sure to do the proper air purge procedure after refilling the system with coolant.
Re-attach the radiator cap, but only to the first click. Start engine and run at idle until it goes past the middle (the RM in NORMAL) and goes back down. Let the engine cool (2-5 hours or overnight). Top off the radiator (an inch or two below the top so it doesn't spill over) and overflow reservoir (to full line only) and again attach cap to first click only, start engine, idle until it needle goes past middle of gauge and drops, then let engine cool again, top off radiator and reservoir, close cap to second click, drive and check for proper operation.
If you're lucky, the heads won't have cracked and you won't have a blown head gasket from the overheating.
Don't ever let the engine go into the red. When it starts climbling past the L in NORMAL and keeps going...shut it down. Turn off the engine even if you have to pull over in traffic to do so.