Envy and anger - why?

Life seems so unfair so much of the time.
I had an OB appointment today. Just a visit so that they could tell me the  results of my blood test that was taken to see if I still had healthy eggs. We've been trying for 9 months to get pregnant. Not a long time, I know, but our daughter and our son were conceived on the first try. Also, now that I'm 43 I really feel that clock ticking. Kai was supposed to be our last. A girl and a boy. Two children who would hopefully help each other out and keep each other as family when we were no longer here. I do consider myself a mother of two, but as my daughter said, "I want a brother or sister that I can talk to and play with.". I understand.
This afternoon I just found out that a friend who is the same age as I am is having her 5th child. They thought they were finished, but life happens. I'm very happy for her but at the same time I'm sitting here crying, feeling like a loser who can't keep a child alive and who can't even get pregnant. Is something wrong with me? Is there a decree somewhere that says I'm only allowed to have one live child? If there is, I wish someone would tell me so that I don't go through all this trying to conceive, dissappointment and heartache over and over again. I feel like I shouldn't even try to think that I can get pregnant and have a healthy child again. Is it because I am in art? Have I poisoned myself with all the fumes from ink, dust from paper? Why?
WHY did my son DIE?
WHY can't I get pregnant as easily as I did the first two times?
WHY can't I get pregnant NOW?
Why do I feel like I am cursed?
Have I let my family down?

i know that these feelings are considered normal, that any emotions I'm feeling are there for me to go through and experience and that they will probably pass in a day or so... but it still sends a stabbing pain to my heart that makes me want to rip it out and stomp on it so that I don't feel it!!