Remember this too shall pass.
I can relate. When I was a stay at home mom and babysat I eventually got stressed out. Other people were always telling me they didn't know how I could do it. I wish I could go back to that time when I could do it. Now I work at the school district. It is not very stressful and I am home when the kids are home. I am also going to school part time to become a medical transcriptionist. I realize this does not help you. My oldest has had heart problems and still does, but is on her own now. I have one at home with ADHD. She has been a trial.
You are an angel to take on the added stess of an additional family member. Try to get help from your church family. I'm sure they would be willing to help out while you and your husband spend some quality time together. There may be other resources available to you also, like Head Start.
Keeping you in my prayers