I've been trying to have a baby for 2 years I missed three periods and took a test and it was negative why is it so hard for me to get preganant?

A Doctor who examines both you and your husband can give you this answer. Many factors could be invovled.

What methods have you been using to get pregnant?
Are you tracking your most fetile times ? Are your periods regular ? Have you seen a doctor for exams ( you and husband)