This happens because your attention span is very short, it's like clouding of mind with lot of thoughts or sometimes due to slow activity of neurons. Other possible reasons are hypothyroidism, stress etc....
So...according to root cause, treatment depends.
There are herbs which enhance memory power and brain quick functioning and there are some pranayam which boosts memory upto great extent
Next Steps
consult privately for complete guidance
Health Tips
Anulom vilom pranayam helps a lot in this...but it should be in proper ratio.