Hai madam, I'm very sorry to know this... I can understand the way you feel. There could be 2 reasons internal or external. I mean to it could be intelligence, memory etc as internal. Or parenting , poor simulation from environment, poor school environment , family situation . Rarely both.
Without wasting time go a clinical psychologist who really deals with these issues and knowledge indepth.
Next Steps
Plan I.Q test , social quotient , Child apprepception test ( just in case any Emotional problem ) dyslexia screeing test, hand writing observation ( in maths , languages etc. )
Health Tips
Don't worry madam things will be ok , when a proper diagnosis is made, because it is the axis for your child's future.