13th IUI in 4 years failed

I'm broken. I've had 13 IUI's since 2013 --- 11 of them with my husband before finding out he has Azoospermia. We decided to move to donor sperm. Well yesterday I found out my second iui with donor sperm failed. I don't get it! I have no fertility issues, I'm 28, my cycles are always exactly 27 days ever since I was a teenager. Why isn't this way working? Our donor has incredible counts/motility, I've responded with 2-4  mature follicles both IUI's and still nothing. I want so bad to give up but I can't. My heart is broken and I can't make myself say 'enough is enough' so I know it's not time to give up, but man. How much more pain do I have to endure? Financially we've destroyed ourselves in the last four years with all the money we've spent on treatment. I'm just tired. And sad. So incredibly sad. 😔