Let's assume that you cannot trust your thoughts.

But, if you cannot trust your thoughts, then you cannot also trust the assumption above.

And, if you cannot trust the assumption above, then why posit it? Instead, posit the opposite - that you can trust your thoughts.

N.B. I find trust to be a someone vague term, less suitable here. Had you used the notions of your thoughts being true, it would have worked in a similar way, and would be a proof by contradiction.

Let's assume that you cannot trust your thoughts.

But, if you cannot trust your thoughts, then you cannot also trust the assumption above.

And, if you cannot trust the assumption above, then why posit it? Instead, posit the opposite - that you can trust your thoughts.

N.B. I find trust to be a somewhat vague term, less suitable here. Had you used the notions of your thoughts being true, it would have worked in a similar way, and would be a proof by contradiction.