just want a baby

Me & my fiancé are getting ready to share our one year anniversary tomorrow But we have been actively trying to get pregnant for about 8 months. I was on the depo for three years & came off in April. I check for a positive ovulation test every month & have sex constantly but nothing :/ my fiancé already has two kids from his ex wife & it bothers me at times that I can't give him a kid but she could. I feel so down about myself because I'm like what's wrong with me?? Why can't I conceive?? I know he wants another one so bad but ever since I had my miscarriage at 17 in 2011 they told me I would probably never get pregnant. All I want in this life is a child of my own. I feel like my faith is going right down the drain. I just don't know what to do anymore but I love him for being so patient with me & telling me it will happen when the time is right. I know for sure he is the one I will be with forever <3