1997 Cadillac Deville iM very frustrated

I have a 97 deville de elegance 2 months ago my very low refrigereant light went on, next my ac/compressor off light went on. I thought it was just low on freon, so I went to autozone got can of refrigerant with gauge, hooked it up and it said full.1 week later check engine light went on. I took it to autozone, they did pressure test and diagnosis on air cond, sai dit was fine. Told me check engine light was a sensor. So I continiued to drive car. Next thing my gas milege started to drop, took it took different mechanic, and he said I had a misfire, and he told me I needed a tune up, he also said he added half acan of freon. My ac/compresor and low refrigerant light went off, but check engine light still on. I continued to drive cat for 1 more month until I started to smell that radiator overheating smell. I watch on my dash the temp gauge would flutuate on freeway to 199 and on side roads would go up to 225. I took car yesterday to differnt mechanic, who told me I had ablown head gasket and wanted 4000 to fix. The car isnt worth that. Im not 100 percent sure he is right. He puts on my receipt that they pressure checked radiator and it wont hold pressure he also told me the engine cooling fans wont turn on and they added adye and could not find any external leaks he said they smell coolant at the tailpipe and that the coolant is milky. As soon as I turn on my heater controls back comes the ac compressor and very low refrigerant llight can anybody tell me whats really going on here? Does it sound like ablown head gasket or not? >