Forgetfulness, easy Fatigue, and inability to Focus, are the 3 symptoms you have mentioned here. However, you haven't mentioned the duration since when you have been experiencing these symptoms.
If it hasn't been for too long, it might probably reflect some strsseful event or situation in life, which you aren't able to cope up with.
However you have been suffering from these, continuously for more than 2 weeks, it might indicated you are suffering from Depression (which is easilt treatable).
Other causes could be Vitamin deficiences (Vit B12, Vit D), Thyroid abnormalities.
Therefore, I recommend you strongly, to get yourself evaluated for the above, and consult with a Psychiatrist. Trust me, you should not take these symptoms lightly, and think that they'll just pass. With the right advice and treatment, you'll be as energetic and focussed as before.