Either is okay, but I'm so used to "before" used as a preposition that I tend to look for an object. "I'm sorry for not telling you this before (what?)."
I know "before" means earlier in time. That said, when you're referring to a measureable time, "earlier" in time is more descriptive. We don't put the cart before the horse, but we are sorry we didn't think of it earlier or sooner. Mayb