It is probable that your daughter is retarded. If you live in a place where a Child Psychologist can be reached, you might get a second opinion. Also find out if there is an institution where such children with special needs are educated. She will benefit from systematic education in a Special School as most regular schools tend to ignore children who are slow. Your daughter will need to be taught at her pace of learning and with patience. She could be taught to take care of her own needs and to be sociable first and then gradually move to simple concepts.
It would be best if you both had some genetic counselling. Find out where such a service is available. You would feel reassured by such a test.