I'm a medical student,and having a strong memory is very important for me,but i'm feeling like my memory is not as good as before years and i forget things, what could be the reason? before my memory was much better?

A few considerations: First, do not expect Alzheimer's, Frontal Dementias, or other degenerative causes. Secondly, do consider sleep disorder, emotional issues, thyroid problems, medication adverse effects (if pertinent), vitamin deficiencies (like B-12, thiamine), prior head injuries, infections (syphilis, HIV), toxins (like heavy metals). See an internist at your school to facilitate.