It is the convention to say that what you are is what you think. Alas, this misses the point in two dimensions.
The number of times I have been told that I think too much and that my problems are in my head. True on both counts. But the implication that the problems would simply go if I stopped thinking too much is delusional.
The 1st missing dimension is that it is not so much what you think as how you feel. If the World and your thoughts generate intenese emotions, then the problem is not the thinking as such – more that you are emotional reactionary. And remember, emotional reaction tp a situation always precedes conscious awareness – we are programmed this way.
My friend Nick, for example, is emotional over-sensitive to expressions on the faces of others. Because the emotion is strong and immediate, he tries to work out a reason – why has his mind reacted so strongly?. He tends to read into these expressions much more than is often there, simply because his inappropriate emotional reaction is raising the expression to too high a status. So his thinking is not the cause of his misplaced reaction, but the consequence of his handling of the emotion, which is in turn amplified by his thinking.
Of course, he can compensate for this over reaction, but this is very tiring and easily forgotten.
The 2nd dimension missing in this glib statement is that what you think now is but a dip in the ocean of your life time of thoughts. Great chunks of the lifelong stream of thoughts, entangled with associated emotions gets stored in your subconscious. So it is very much more what you thought in the past rather than what you think now that defines you.
Whilst you can leverage change to the mental and emotional weight of this life time of memories, for example, via cognitive behaviour therapy, or a more positive, accepting attitude to life, there is a lot to reverse.
For example, I can be awoken, as last night, by deeply anxious feelings. I often awake to these feelings from relaxed, delightful dream states – the contrast is enormous and always surprises me. The point is that the anxiety is historically caused, bearing no relation to my dream state, nor my thinking, since I was not awake!
In spite of this legacy, we should still try to turn any negative thinking around – to try to counter established bad thinking and emotional habits. But when people complain that what you are thinking now is sole cause for your mental plight – that what you think today defines the whole you - then they are missing two dimensions.