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I am sorry to hear about your difficulties. An intrusion of this magnitude would not be lawful unless justified by a warrant based on probable cause that surveillance would yield evidence of very serious criminal activity. Otherwise the intrusion would be simply pointless and entirely improper.
The difficulty is, as you have already found for yourself through your private investigator's efforts, such surveillance can be difficult to prove. Even you don't seem to know who is behind it or to be able to show the authorities evidence which would allow them to see for themselves what you are experiencing. They cannot stop what they cannot see and cannot hold anyone responsible for.
Until you can show irrefutable evidence that what you are complaining about is actually happening to you, all of the agencies, organizations, and people you would try to contact are going to turn a deaf ear to your problem and tell you it's a mental health problem. So, you need to start collecting evidence that others can see as well as you see this for yourself. Until you can provide the authorities with this concrete evidence that you are a target of surveillance, nobody is going to be in a position to stop this. You should be taking photos and videos and/or recordings, when you have evidence of this surveillance for example. You can put an inexpensive surveillance system in your home that's responsive to sounds and movements. When you're armed with sufficient evidence, reporting these activities to the police or FBI should get you some relief.