"Sorry, I can't think straight right now", "My mind is all over the place", "I'm mentally tired", "I'm shattered, mentally", "I'm mentally exhausted right now, I need to rest a bit"
There are many ways to say it, such as those examples above, and nothing is more accurate & efficient than "my mind is f*cked" but this phrase is blasphemous so should only be used if you are in the presence of people who you know are comfortable with such potentially rude words.