My heart goes out to you. I suffered from infertility as did my mother. So I understand your sense of loss and incompleteness.
I have two suggestions. The first is that you work to strengthen your immune system with a comprehensive liquid nutritional supplement. Do not mess around playing 'kitchen chemist" with individual vitamins. Do not start doctoring yourself up with herbs because they are "all natural" - so are arsenic, snake venom and poisonous toadstools, you know? You want a patented product made in the US with a great FDA inspection and proven results. I have a lot of friends who have had great results and successful pregnancies despite all kinds of fertility problems - and several whose doctors marveled at the health and thickness of their babies' umbilical cords at birth. Those babies got lots of good nutrition and a great start in the world! I would be happy to have you talk to them about what they did.
Secondly, I have no idea why your doctor is saying you are "too old" at 36. I was older than you, and many women in their 40s are having successful pregnancies. You need to get another doctor NOW, particularly one who specializes in high risk pregnancies. You should have a blood work up to see why you cannot carry a pregnancy to term. I think you can safely skip some infertility tests (such as checking your Fallopian tubes to see that they are clear - they obviously are) but something like a uterine biopsy might make sense - depending on the results of the blood tests). You are getting pregnant but for some reason you are unable to sustain a pregnancy - so it could be hormonal (insufficient support to the lining of the uterus/placenta) or something else. If there is a good fertility clinic in your city, I would start there. If you need a referral from your OB/GYN, insist on it. If that doctor won't do it, get a new one. Meantime, see if there is a local chapter of Resolve in your area - they are a support organization for people dealing with infertility (and many women are in your situation, having had one or more successful pregnancies but now having difficulties) - they may be able to help you with doctor referrals as well.
My belief is that God gives us the resources to help solve so many of our own problems - our brains, our hearts, brilliant scientists and medical advances, and the free will to avail ourselves of a whole array of services. To sit back and wait for divine intervention makes no sense - maybe your faith with make you strong enough to pursue other avenues on your own. There are other doctors in the world, but there is only one YOU.