The 4th Miscarriage...HELP!!!

My HCG was tested 137 on 11/30, and dropped to 37 two days later. The nurse told me it means this is not a healthy baby and we need to let it go. But this is the fourth or fifth miscarriage I had. The first miscarriage happened in Jan 2019. It has been almost two years. Every time the baby goes away at the 3rd or 4th week. My husband and I did a lot of body exams, blood tests and everything looks good. I keep taking pregnancy vitamins since one year ago. I don’t know why this still happens to me??? The doctor just say the baby is not healthy. But when it happened so many times I think there must be something wrong. Do you have similar experience or any suggestions??

BTW I am 30. Why it is so hard to have a baby...