Trying to conceive.

My husband and I have been trying to conceive for a year and a half now and still haven't gotten pregnant. We have these few months where we try during my most fertile time and lose hope and eventually stop timing out my ovulation cycle and get intimate whenever (hoping luck will just take its course)... After all this time, I'm starting to feel there's something wrong with me, being that he already has a child from a previous marriage. Any sort of advice or words of comfort is appreciated.