If you've ruled that out, here's how to fix it:
Drastically reduce gym time, or stop going altogether, for a week. This way you can allow the body to recover properly, and in turn get ready for more work.
Quit tracking your diet and allow yourself to eat more. One cause of overtraining that actually has nothing to do with the fitness workload is diet: sometimes people don't eat enough to support their workload. By eating more and training less, your nervous system, hormones, and immune system will have the necessary fuel to recover properly.
Sleep more. By cutting down on gym time, you give yourself a chance to get more sleep each night, and this is paramount to recovery. By getting more sleep, you'll recover more quickly, and be able to hit the gym again.