Car overheating and no A/C

The A/C was bypassed after the belt seized up. The coolant light and temperature light come on every once in a while and the temperature got so high once the cap for the antifreeze blew off while I was driving. Running the heater on high can stave off overheating to an extent but it still goes over half the gauge and it is extremely hot! So we replaced the thermostat and that did not help. The antifreeze is not leaking out of the car all the time, it did leak out the bottom on drivers side once, but it does keep getting low and sometimes it is gone. Also, it burns through oil. What should I do?