Loss of interest, alertness and slow thinking are classic signs of clinical depression. St John's Wort has been shown to be effective in mild depression.

Clinical depression is a real disease due to low levels of some neurotransmitters in the brain. Your symptoms can also be found in other medical conditions like low thyroid, sleep apnea and anemia. I know this is a herbalism forum, but have you seen your medical doctor to discuss conventional therapy and rule out other diseases or considered seeing a therapist? Talk therapy is also effective for many cases of mild depression.

Here is a link that might be useful: depression

If other medical conditions (such as hypothyroidism) have been ruled out as a cause of your depression, non-drug/non-supplement options include regular exercise and light therapy. Both may improve mood and thus help brain function in depressed people.

It's a dangerous thing to self-diagnose! Depression can be a condition in itself, or it can be a symptom of any number of other conditions. Get to the root of the problem, and treat that. So, your first step should be to visit your doctor.

Herbs for 'aging' might be helpful if you've eliminated other causes of your poor cognitive functioning. Think of things like Kotu kola (Pennywort), or Ginkgo biloba, or Brahmi (Bacopa moniera). Rosemary is good for the memory.

Do some searching for herbs which 'improve circulation'. With a better blood flow to the brain, brain function will usually improve.

Be careful which herbs you choose. Most have multiple functions, so it's best to consult a suitably qualified herbalist who will be able to prescribe the best ones for you in the appropriate dosages.

Be careful, or you will truly HAVE chervil tea for brains. :D