So tired and frustrated

I know I'm not out until AF arrives but I'm 9 to 10 dpo and have gotten 3 bfns. The man and I have BD everyday of my ovulation and I really felt like this could have been our month. We've been trying since a December chemical and it's just so tiring and heart breaking. I wish I knew what I was doing wrong or what's going on inside there. It's also so stressful that I just want to stop altogether because it almost makes sex not fun anymore and more like work. But I want one last baby so bad (only have one child - soon to be 8yrs old). Anyone else feel like this? Anyone have any suggestions? I really feel like a failure and not worthy of another child...