My daughter is 26 months old and will bump down a small flight of stairs or our larger flight IF I am walking backwards down them in front of her.
My suggestion is to carry her down the large flight of steps until she has her confidence to either bump or belly-crawl down them on her own. Until then, it will only aggrivate the fear.
Have her practice bumping on her butt or belly-crawl down 2 to 3 steps at a time to build confidence. If you have a playdate somewhere where there are other kids (esp. older) have the other child show your daughter how to go down a few steps at a time. It is amazing what littles will try when they see other littles doing something. Monkey see, monkey do.
Keep encouraging with soothing positive words when appropriate and carry her down large flights of steps until she has her confidence.
Good luck!