I'm so numb...I want to give up trying

I have been married to my husband for 4 years now and was previously in a 7 year relationship I have tried having a child in both and every month I am heartbroken. I am the eldest in my family and the only one to not have a child.i just found out my 18 year old sister is pregnant with her first child and my younger cousin. Is having her second child and I can't help but feel bitter sweet.of course I am over the moon happy for them but at the same time I feel so heartbroken and tbh I feel as though I am failing....I can't have a child I can't look into the little oh so similar eyes of my child. I can't take my child trick or treating or buy them presents for birthdays and Christmas instead I get to witness all the mothers in my family interact with their children as I sit on the couch and hide my pain behind a smile, behind excitement. To feel a kick or hear a heartbeat or go through tremendous pain for the greatest blessing a woman could have all things I've always hoped for, prayed for, begged for......what did I do to deserve this? Why can't I be fixed? Why can't I be a mother.... I look around and I try so hard to be positive happy hopeful and yet I seem to be tied between giving up and trying harder. But ik that will not change my fate.... But I have wanted nothing less.