Chemical pregnancy

I've had 3 chemicals/miscarriages in 2 years. I have a healthy 4 year old. Why won't these babies stick?? What am I doing wrong???? Logically I know it's nothing I'm doing but damn....cut me some slack here. I almost wonder what's better...not being able to get pregnant at all, or getting pregnant but miscarrying. The grieving process is horrible. How much more am I supposed to take????