I had gone through something like this when I was 16-17 years old where I thought people were following me, always being watched. I started memorizing license plates to see if cars were always around when I was. It took a good year for me to really stop thinking this way, I unfortunately don't have any tricks to how I stopped it but it did stop. My Doctor had mentioned to me that a lot of people suffer from this at one extent or another. He even mentioned 'Truman Show' syndrome where people believe every bit of their lives are being taped in a secret reality show.
Though if this has been going on for an extended period of time I do highly recommend talking to your Doctor about it and seeing what he/she recommends as back when I was suffering from it my Doctor gave me some breathing techniques to try but I believe understanding of this syndrome has improved since I was 16-17.
--Jess