My answer actually disturbs me.
I take Wittgensteins argument against private language as successful, and I take Quine's argument that language is empirical as successful.
Here is an example from Wikipedia's article on Wittgenstein's private language argument:
"For instance, if one cannot have a private language, it might not make any sense to talk of private sensations such as qualia; nor might it make sense to talk of a word as referring to a concept, where a concept is understood to be a private mental state."
Here is a quote from Quine's essay "Epistemology naturalized"(Quine1969):
"Language is socially inculcated and controlled; the inculcation and control turn strictly on the keying of sentences to shared stimulation."
I'll add that there are languages that omit concepts contained in other languages. There is a language, for instance(I'm sorry, I'm going on memory. Can't find a reference) which describes time as passing us from behind going forward. Like a wind at our back. We see ourselves moving forward, time passing us like a wind in our face. There is also a language (so sorry, no reference) which does not have the concept of contradiction.
I take all this to mean that the formulation of our thoughts, and, indeed the concepts it occurs to us to think about, are given to us by our language group/sub groups. Wittgenstein and Quine are saying we cannot express our thoughts independently of society, and the varying conceptual content among languages signifies that even what we think about is determined by our group.
When we think about something is often determined by the context of the moment. I cannot argue that we determine a type of concept to think about, nor how to express, even to ourselves, unspoken, the concept.
Pain is the classic example of private experience, which cannot be accurately expressed to others, in language, because we do not have access to how others feel when they experience it. But we do not control the experience of pain. And when we do think about it, I think we use the public language.
I'm not happy about it, but I only know of arguments against individual determination of thoughts.