Well, first make sure your problem is not a neurological one. Check it out with a neurologist / psychiatrist. If it is not neurological, then it would appear to be a memory RECALL problem. The memory is there. It you are unable to recall it. This can happen to those whose minds are in a state of frequent distraction. Your ability to concentrate is weakened. This your ability to assimilate, learn, memorise, are also weakened. What often helps is a regular practice of yoga and anapana meditation ( find it in the web). Therapy may also help.