Its going to be your cooling system, it sounds as if you've boiled over twice. Could be your water pump, water pump pulleys and or your thermostat.
With luck, your engine has gone to a reduced power mode to protect itself from seizing.
When replacing coolant, you need to get all the airlocks out of the system. There's loads more than just the expansion tank. Once topped up, before turning the engine, squeeze any of the water hoses you can find, if there are air bleed valves, open them until water comes through.
Have plenty of spare water with you, turn the engine over with the lid off the expansion tank, also turn your heater on, max heat. This will allow air to circulate out of the system. You may have to go back to the hoses and bleed valves a couple more times. Top up as needed. Finally, check your
Even after you think you're done, keep an eye on your coolant level for a few days, there still may be an air bubble in there.