You have to understand that many of the things you take as certainties are actually unknowns.
We know really little about the brain still, we don't even know what it is, does it create consciousness or does it receive it?
Because it is still unknown to us what reality is, what consciousness is and what the mind really is
The same goes for mental illness. To what extent are we creations of our brains and to what extent is our brain a creation of our mind?
Research show it's not a one sided answer, as changes to the brain through alteration can have gigantic effects on us.
Yet at the same time, we can change our brain through our mental will power, i.e. look at studies done for OCD patients who changed their brain to a more healthy looking brain simply by the power of their conscious will.

But if one thing is certain, stop messing with your bodily chemistry all the time and learn to appreciate life without drugs, learn to calm down the endless stream of thoughts and emotional unrest, learn to know what it is like to just be happy and at peace by just being with yourself. That's right, just being happy and at peace without doing anything. It's possible.

Look into the eastern philosophies, where people have studied this practice of cultivating peace for thousands of years, on how to be happy through the simple act of detachment and presence, sometimes even in the most dire circumstances (these practices are known as meditation and mindfulness).
Speak with advanced practitioners, see what their presence is like, ask them if what I am telling you is possible.

And not only are there the testimonies of thousands or even millions of people, currently there is also a lot of science behind it.
Here is a good list of peer reviewed studies, and it features a mere fraction still.
www.reddit.com/r/Meditation/comments/2i875y/meditation_will_improve_your_life/ (that sub reddit is also a good place to ask around and read up by the way).

You were happy as a child when you didn't have to worry and you can have that same happiness now when you start to calm down.
Then, when you are calmed down, you and can be happy without drugs, you can explore psychedelics again. But you might find you don't need them anymore, do you know that the most powerful psychedelic actually gets created in our bodies naturally? It's the chemical called N'N-DMT and it makes people experience 'the impossible', what ever that may mean, because no one can tell you what that is like except a direct experience, this is a nice compilation video of what that experience is like:

Peace, take care my friend ;)