1995 Plymouth Acclaim Loosing coolant

I recently have been loosing coolant from my car b/c it always drops to the min level in the reservoir tank. I have to add coolant every 2 or so days. I had the car pressure tested, diagnostics run, they checked the water pump, had the thermostat changed. It still keeps going to the H or even past it. When I start it up a lot of white smoke is coming out and then when I am driving for awhile it doesn't do that. I also when I stop somewhere and come back after like 10 min to start it up the car struggles to start up. Sometimes it idles and runs rough for example when I am at a red light and struggles to get going but after I start accelerating it runs okay. Also I have no heat either. What should I do next. I am not leaking coolant. Could it be the water pump? Head gasket? Manifold? What should or can I do on my own and what should a mechanic do and how much money should I invest into this vehicle?