Not to get all he*&-fire and brimstone on you, but satan is real. He and his demons have come to steal, kill and destroy. But I've known that a long time. And like you, I've been feeling the effects so much during this time of my life.
Here's a great question: does God 'allow' suffering to 'make us stronger' or 'grow our character'? I don't believe He does. He is good; that is His nature and He cannot go against that. Jesus came to earth and died for our sins; His love is good and perfect. Any time someone came to Jesus and asked Him for healing in the Bible, he gave it to them. He did not say 'no - get right with me first' or 'no - I want you to learn to bear up under it'.
So why is all this happening? For the millionth time asking myself, I still have no answer. However, I have some peace knowing God did not decide this was good for me and bring it into my life. Infertility and miscarriage - each loss we have endured - is the work of the destroyer. There is no infertility in heaven. Just as there is no cancer, no death of any kind.
I believe that I am living in the in-between: between what I can understand and what I don't understand. The why of what I'm going through is one of the things I don't understand. God is giving me the grace each hour of each day to live in the in-between. And as we all know, it is not easy. But He is here, and He did not cause this - He did not 'allow' it to teach me a lesson. He didn't forbid it either, however. Because all along He has a plan to work for the good of those who love Him, who have been called according to His purpose (Romans 8:28).
Let me also say that God has been misrepresented to me alot in my life. My father was a very angry man and took this anger out on my family and me during my growing up years. He is vastly different today, but our relationship is largely broken and stale. I am unable to pursue a healthy relationship with him to this day. We get together at holidays, and do the usual things, but it is largely just a walk-through instead of a real, living relationship. I say this to illustrate that my template for our heavenly Father, Jesus, was largely colored - and colored in error - by my relationship with my earthly father. So this knowledge that God is good and just how good he is is a really big deal to me.
I know it's just arguing semantics to say did God allow it or did he just not forbid it. And this is a great mystery. I do not have the answers, but I do look to God for them. Many of the answers won't come this side of heaven, but I have peace today knowing that God is good.