TTC and not happening

My husband and I have been TTC since the beginning of the year. We have 3 kids already 2 girls and 1 boy 10yrs 6yrs and 5yrs of age. We are aiming for a boy. I'm 29yrs old and I will be getting my period in a week and couple of days (hoping I don't) we also purchased the preseed since I've read wonderful results when using it. I took a pregnancy test came out negative. I've been taking prenatals to prepare my body. I have been thinking on giving up its frustrating. :(