Counting is very difficult unless you started on day1.
Sometimes it's a little easier to understand the difficulty by knowing what it is exactly that you're asking your brain to do...multi-task!
Your brain is a finely-tuned, digital computer...it can't do more than one digital operation at a time...it can only appear that way by switching digitally between multiple tasks in a quick enough manner that it appears things are happening simultaneously. If you're "thinking" about the timing, saying it and playing it at what appears to be the same time, that's a tremendous amount of "switching" going on in your brain to make it a appear simultaneous.
It just takes a lot of practice. If you're like me without a shred of "guitar DNA," it takes
lots and lots of practice.
I just figure if it were easy, everyone would be a Clapton or Bonamassa...it is tremendously fun, but it isn't easy.