Hey M.,
I felt like I had to respond to your dilemma because I have such a similar situation (in some ways, at least). Except I think I'm a bit older than you with a few more kids than you have.
I got pregnant when I was 40. Needless to say, it was a surprise. I, too, was charting (do you do NFP?), but I guess my body's changing as I get older. At the time, we already had four wonderful children, with the youngest being 8 years old. I thought God was finished sending us children so, while my husband was away on a weekend retreat, I thought I would surprise him by cleaning out the attic and giving away all the baby and toddler clothes and equipment that I had been storing. THE NEXT WEEK I GOT PREGNANT! God certainly has a sense of humor :)
Well, after we got over the initial shock, we decided we were thrilled. We would have a baby in the house again, and the older kids would have a chance to help raise and nurture a younger sibling. By the way, we also homeschool (just finishing our 12th year), so the kids are home to help me all the time. I should insert here that I have a house full of boys, ages 18, 15, 12 and now 3. My only daughter is 21 and a junior in college. She helped raise all the boys until she moved out.
Anyway, our baby (now toddler) has been such a joy and a blessing to the whole family. Altho I'm older and stiffer and more tired, God gives me the grace every day to do what I need to do to care for and love all of my children. And the kids really do help me and each other. It's a pleasure to watch them interact with and teach each other. Not that it's always paradise, but the joys certainly outnumber the trials. It also helps that I'm married to the most wonderful man in the world, who supports me in home schooling, mothering, NFP, and basically everything I do.
The other similarity in our situations is that I've got a little one in heaven, too. Six months after this surprise baby was born, I got pregnant again. But I lost him half-way through the pregnancy. Our three-year-old would have been 18 months older than this baby. I really wish he were with us, but I know he's praying for us from heaven. I even ask for his help occasionally, and I know he has God's ear.
So, my advice to you would be: PRAY! If God has decided to bless you with another child, know that He will give your family the grace to raise and love this child.
--Look forward with joy to the friendship this new baby will share with your next to youngest child as they grow older together. And think what an opportunity this will be for your five-year-old to feel grown up as she helps you during your pregnancy and with the babies. (As I said, my daughter helped me raise all her brothers. I had each of the four children about 3 years apart from each other.)
--Communicate with your husband. Affirm his role as provider for the family and caretaker, and as your perfect partner. Encourage him to take the kids out for some "bonding time" when you need a nap or a break. Remind him often what a good husband and father he is.
--Know that you're not alone. In our Catholic homeschool group, there are lots of families with 8-10 kids, most of them two years apart or less. They seem to be doing just fine. If they can do it, so can you.
--Know that I'll be praying for you. And pray for yourself and your family. Ask your child in heaven to pray, too. She probably already is.
God bless you :)