Repairs to the aging Sagamore Bridge throughout the spring have slowed traffic leaving Cape Cod to a crawl most nights and backed it up for miles on Sundays, culminating in a Mother’s Day morass when the stalled line of cars stretched past multiple exits on Route 6 and triggered all-day gridlock on nearby 6A. Officials said the work, to repair and replace many steel elements, is necessary to extend the life of the nearly 80-year-old Sagamore and will wind down before May 24.