To start I have three children 6, 4, and 9 months. I had two miscarriage between child 2 and 3. The second miscarriage was a set of twins. So I'm not totally clueless about what you are feeling.
It is natural for many of us to be jealous of others when they have something we wish we had. It is natural, but it isn't right. We have to learn to except things we can't change and beat down that big grean monster that torments us.
1.) You have one beautiful, healthy child now. Concentrate on that one. It helps to fill the void.
2.) Being jealous of others really hurts us the most. You are dwelling on serious negativity.
3.) If you have faith in God, then you need to get into your Bible and read what it says about coveting, jealousy, and love. It will help you a tremendous amount.
It just isn't the time for a new one. For whatever reason I'm sure it will all work out for the best in the end.
PS After my second loss my doctor ordered a battery of tests that included some genetic testing. I have a mutation called PAI 1. It causes blood clots in my uterus when I'm pregnant. I guess it doesn't manifest itself all the time since I had two normal pregnancies. But I had to inject a blood thinner with this last child to term and he's great.