If the inability to think stems from the thought “I am unable to think”, then OP might be better off with written CBT tasks than just writing in general. That’

If the inability to think stems from the thought “I am unable to think”, then OP might be better off with written CBT tasks than just writing in general. That’s pretty much doing what you suggested but with a clear instruction set on how to. My experience is that under certain conditions your mind might not be able to get free writing correct in a way to make it really help you. Then, it might further discourage you from trying even though all that was wrong was not missing thoughts but habitual negative thoughts preventing you from doing what you want to do to begin with. This can be quite tricky so it would be very helpful to learn more details about OP’s exact issue.