My wife is too tired for sex

I’m scared. I’ve been married for seven years and thought I had a fairly good marriage until around the time of the birth of my second daughter. Now I am wondering whether our marriage will survive. My wife is too exhausted to do anything in the evenings; all her affection is focused on our children. She has no desire to let other people look after our kids so we can spend time alone and she is too tired for sex. I’ve realised she has never given me the affection I have been after and our relationship seems to have been based on our first child since she was born. I have worked hard at our relationship and prayed for it over the years. I don’t know what else I can do and I’m beginning to think that we might as well divorce. We have talked about some of the issues and it’s good to know my wife has an understanding of how I feel, but no improvements have occurred and my hope has vanished.