Tl;dr: I highly doubt that's the source of your problems.

Oof, man I'm sorry for your experience and I'm sorry the quality of replies you're getting is so.. Shit.

How do you know you've been inhaling this for so long?

Burning plastic can be bad, in addition to the chemicals "Remotelove" noted, there's a whole bunch of potential organic compounds created in the burning process. Exactly which ones depends on the type of plastic, temperature and how much oxygen is available. At complete combustion you'd basically just get CO2 and O2, but at less than optimal conditions you can get a variety of products (think ash). I'd be mostly worried about any carcinogenic products though, since a significant amount would be PAH's (https://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbon).

Now, for something you ingest to have a long term effect on your brain, it must (generally) somehow be able to get into your brain and your brain is pretty well protected by the blood-brain-barrier. Hypothetically a non-polar product from the burning plastic could cross over into the brain and mess shit up, but the chances of that are slim. And you'd also need to inhale enough of the smoke, for there to be an effect.

Listen, I'm not saying definitely not, but I'd be very surprised if that was the case here. I highly doubt that you have been breathing in thick, black smoke, so the amount of "brain damaging" molecules you've been breathing in is probably pretty low. Mutagenic or carcinogenic, sure possibly, but I wouldn't worry too much about that either. I imagine you've inhaled way less than if you were smoking a cigarette daily.

I suggest you try to think of other causes, a lot of things can play a part in "feeling more dumber er", like depression, lack of sleep, stress, unhealthy eating, lack of physical exercise etc.