K.,
As a child, I lived in a one story house. My father took me to bed on his shoulders, gave me company to go to bed, tucked me in, etc. You know, everything seems so big at that age. I can easily imagine how a 5 year old would feel that upstairs is way too far away to be by herself at night. I think she should still be put to bed with company, a story, etc.
Is your husband trying to manipulate you by saying things that might make you feel foolish for wanting to make a 5 year old feel secure? She's only 5 for pete's sake! You don't treat a 5 year old like a grown up.
Aos, did you perhaps ever share your long-standing fears with her? Might she think that if 'big strong mommie' was fearful, she has a good reason to be fearful too?
Why not give her reasons to feel safe. Be with her at bedtime. Be with her when you tell her to turn on the lights (if she can reach them), give her funny, loving memories to go to bed with, use some funny but educational bedtime stories. Include some that teach lessons (like it's common for children to be afraid of the dark or doctor or dentist or creaky sounds in the night -- but they are treated with a gainful trust in your being her absolute protector and that the house os a warm and wonderful place. Is it? Are you guys coming off as parents to a be fearful of or that ignore her fears? Does she have a tiny night light? A nice bedtime routine? You don't really want her to be 34 before she's not afraid.
I don't understand your connected her being a "social child" to her being afraid to be alone upstairs at night.
Make sure you or your husband aren't giving yourself excuses to stay on the couch watching TV when your little ones go to bed.
Keep up on parenting skills. Read about it. We think we know -- but without reading and keeping up, we are just following our parents' patterns (good or bad). Some parents wait until their kids are in trouble and some other adult tells them to learn how to be a better parent. Don't let that be you.
Ever watch "The Nanny?" That show can typify how parents are totally responsible for the way their kids turn out badly at that age -- and how to turn it around. Parent participation, listening, guidance, rules, hugs, attention, play times, work times, etc. Be your child's best advocate and develop them into responsible, fun, loving, motivated, kids who feel safe and secure by what you DO not what you say.
When people tell you to just pray about it or teach her to pray about it, think about that carefully. Praying is fine but how many times will she pray for something she won't get? How many good and prayerful people have bad things happen to them? For me, I think we need to be mindful that God gave us a brain and a heart and lots of other people with special talents to do more for your children than sit around and pray for Him to help you or your child. Take personal action and be open to growing and learning for as long as you live.
Lastly, if anyone offers their professional services to you for money, be mindful that they might possibly be using this forum to gain income from you or others -- like advertising. When I read that say a chiropractor wants to give you or your child counseling for a fee, remember that this is not what they are licensed to do.
Good luck. Let us know what works.