My car keeps overheating and my water keeps disappearing or evaporating?? on 1998 Buick Regal

For 5 weeks now I've been having to constantly add water everyday to my radiator....its not visibly draining but the water is going somewhere bc its not staying in my radiator! We changed the heater core and git a new rad cap and now today we are changing the thermostat!! But where can my water be going to constantly make it overheat??

have cooling system pressure tested for external leaks, if nonoe found ck for blown headgaskets