[DJ Full]

I convinced myself to believe things that I was absolutely certain about it.

I was religious for 20 years, in 2010 my faith was so strong that if you put a gun against my head and requested me to deny god, I would tell you to shoot. You aren't really different from what people do. Let's be glad you woke up unlike them.

I really believed (among many other things) that cars were strategically parked in my street or in front of the lab window to observe me

The Killer Cars come to mind... but yes, that can be annoying. I had a window in front of an office building, everytime I turned on the light I knew anything I do was visible for these guys - even though it was very unlikely anybody ever watched. I know how annoying this is, but I just closed the curtains and tried to ignore that.

and coded messages were left for me on the blackboard in the public lobby.

Hah. I remember I had a compulsion to approach the corkboard in our school and use the free pins to draw constellations. I always wondered if someone will recognize them. In any case, out of 99 random arrangements 1 might be intentional and valuable, but not necessarily for you, and not necessarily a threat - sometimes messages are just messages. You can try to decode anything what attracts your attention and not worry too much.

Help, what have I become? I used to have a good bullshit radar, and now I'm bullshitting myself into believing things.

From what I noticed you're getting better. Was I wrong?

now that I'm so painfully aware of my flawed brain, I fell into overthinking and anxiety limbo

This is the same place where I am now, so I cannot tell what will follow. But I just try to do what I'm supposed to. It's good to occupy the brain with something more so it doesn't have time to worry. It's hard but possible. And much easier if there's a deadline

I guess I have to be practical about it, especially with the stalking. Since I don't have any evidence, there's nothing I could do about it anyway.

Oh yes, you can. No matter if there's any stalker or if there is none, act like if there is none. IF there's none, you will get used to ignoring him, in fact ignoring your fear. IF there is some, he will notice you don't care and he'll make that one step closer which will give you the evidence you need. Either case, ignoring is the way so at least there should be no dilemma.