I can totally relate to what you've said. I started questioning the worlds existence, my existence, everyone else's etc a long time ago. I don't remember why, although I do remember one time (I used to use cannabis, I don't remember if I was at this precise time) when a friend of mine was studying philosophy and was telling me about philosophers theories like we could all be just brains in jars and such. This played on my mind an awful lot. Thinking about the universe plays on my mind, I find it hard to imagine it, never ending. I wonder why we're all here, what's it all about? Do we actually have a purpose that we're unaware of? Is there such thing as fate? Is there even a reality or is it all made up?
Sometimes the words I speak don't seem real, thoughts don't seem real. It's really weird! I can't really explain it.
I don't really have any advice, apart from maybe not using cannabis (if you still do) and see if that makes any difference, perhaps see a counsellor? They can help you see things in a clearer light. Maybe the cannabis triggered something off, or maybe it would have ended up like this anyway, who knows. But I would give it a miss for now, I know I began to suffer quite badly with anxiety when using it before, it never used to do this but it did a few years back so I stopped and never did it since. Obviously I don't get cannabis induced anxiety/panic, but I do still suffer with anxiety and always have even before using it. All I can say to that is its playing Russian roulette with your mind, because its not regulated here, you never know what you're getting.
Sorry I'm not more helpful to you, I do hope you can find a way to deal with this. I'm here if you want to talk