frustrated

My husband and I are trying to have a baby of our own. Don't get me wrong i already have two. I first got pregenant at 17 and i wouldn't change it for the world. Two years later my son came along. There dads both bailed out on me. Now I have a great husband who loves my children completely. So does his family it is just that we want another one.
We have been trying for almost 18 monthes we have lost one and now it is like we just can't get pregnant. His family is all excited about us trying. But that just makes it worse.
He blames himself because i have had two babies and we haven't had one yet. The Dr. says just wait it will happen but it is takening forever. I just don't know how much more i can take. Things aren't the same since we started trying and a year passed. It just isn't fun anymore.
It just seems that when I finally have everything in order and going my way that i can't get the one thing that i want; but when my life was horrible icould and didn't even want it at the time i was still a kid. Pretty depressing if you ask me.