Tough time TTC

I’m so discouraged. We miscarried last October and haven’t been pregnant since. I’ve had ultrasounds and everything is perfect. I’m in good health reproductive wise. I just don’t understand why I can’t get pregnant then. I’m ready to give up and just say maybe it’s not meant to be. I have an amazing, handsome 12 year old and I’m super thankful for him. Maybe he’s meant to be my only child. I feel like I let him down though because he wants a sibling so bad. I feel like an awful mom, but even more less of a woman because I can’t conceive a child.