I am so sorry you're going through this. I wish we could give you a concrete answer, but sometimes, there just isn't one. I would advise asking your OB to run various tests to make sure nothing is "off" with your body, hormones, etc.
A friend of mine had a healthy baby, followed by two first trimester miscarriages (one happened naturally, the second lost a heartbeat but didn't pass on its own). Her doctors confirmed that she could easily get pregnant but couldn't maintain the pregnancy. She saw an acupuncturist and an herbalist who each specialized in fertility for several months. She made changes to her diet and her environment. After several months of treatment, she and her husband started trying again and ended up with a healthy, full-term boy.
Obviously, alternative methods aren't for everyone, but it's worth considering.
Sending hugs and good thoughts.