2 years TTC.....

I have 2 boys with my husband. August 2015 we decided to try for a baby as our youngest was 6. I had a miscarriage in 2012 and since then was on birth control. Mind you, I was on birth control when I conceived then. It's been two long years with 7 rounds of 100mg clomid and lost hope. I don't know what else to do. I have thyroid cancer, and have had a thyroidectomy with ritual cancer remaining. I've been taking my meds as instructed and been doing all I can. I guess I just don't understand how it is that I can't get pregnant it's heartbreaking. I'm very thankful I have my two boys, and will keep trying. How long did it take you??