I've smoked marijuana in the past and done psychedelic drugs, but this was all a while before the spirit first appeared. I've read that marijuana use can weaken one's energy body, don't know if this is true, but maybe that played a role in allowing me to become possessed as per lama dawa's explanation.
Nothing strange happened in meditation that I can recall. Just started out of the blue last January
It might be the drugs that you have done in the past as drugs have great effects on the brain. This would cause the brain to function not normally. So, it might be a brain problem. That's why you are hearing voices and such. Do you find that you cannot control your thinking sometimes? Responding erratically mentally to certain situations without have control over it? And stress would definitely increase this situation.
If you are high on drugs while meditating or doing drugs and meditating, you will definitely have what you described. So if you are still doing drugs now, I think you should stop.
You think it is a spirit.
Anyway whether it is a spirit or drug related, I suggest reciting Namo Amitabha over a couple months will definitely help. But you need to have complete faith and be consistent and persistent. Even if you are distracted at first from reciting Namo Amitabha, please continue to do so regardless of how the spirits or symptoms bother you. Trust me you will experience changes from inside out, and find yourself more focused even if your recitation is not good at first, but over time you will be more focused in your recitation. You will have to wrestle with intruding thoughts at first, especially those thoughts. Namo Amitabha can cure your situation but you have to have faith, be consistent, and persistent. Use Namo Amitabha to subdue all the craziness that is going in your head including those intrusive thoughts when they arise. There are many versions of Namo Amitabha for examples Namo Amituofo in Chinese and Nam Mo A Di Da Phat in Vietnamese. You can use any version you like.
NAM MO A DI DA PHAT