When you are tired or distracted, you sometimes feel that your brain is not functioning well because of lack of focus. Many years ago I said that I'm absent-minded, but a native speaker of English said that it sounds like I have something wrong with my mind. But that is what a dictionary says. Then I have found these phrases, "My mind is scrambled." "My mind is foggy." "My mind is fuzzy." However, I don't know if these phrases are appropriate. Can I use one of these phrases to express what I want to say in a daily conversation? Or if there is a good phrase for this, I'd like to know.
You can use many phrases based on those words, usually they will depend on exactly why you're struggling to concentrate.
They mostly mean the same thing but sometimes convey another meaning:
"My mind is scrambled."
Perhaps you have been working too hard, or trying to work on problems that are too complex for a long time.
"My mind is foggy."
Perhaps you are very tired or there are some drugs or exhaustion which is affecting your mind.
"My mind is fuzzy."
Usually fuzzy is used to describe imperfect recollection. "I think I had eggs for tea, but my [mind/brain/memory] is fuzzy".
Most people will simply say "I cannot concentrate" as mentioned in the other answer. This is pretty generic, it could be because you are tired, you have a headache, you are worrying about something, or many other reasons.
There are many other phrases too, "My brain is fried" is one of them. Ultimately it depends on exactly how you want to describe the situation, rather than having a situation-specific way to say it.
A conventional expression for this state might be
I can't concentrate.
Where concentrate means to direct your attention or your efforts towards a particular activity, subject, or problem
Rather less formally, you can say
I've lost the plot
If you say that somebody has lost the plot, it implies that they are no longer able to keep up mentally with what's going on around them.