I handle these types of claims for many title insurance companies, and have litigated easements in cases up to the Maryland Court of Appeals (including several in Frederick County). Here are some thoughts:
1. If the easement is in writing, a lawyer needs to read that document and it must be compared to the easement that is "on the ground." Is it possible that the neighbor is not using the easement that is described, but one that has developed in a slightly different location?
2. If the easement is not in writing, then a lawyer needs to discuss with you how that easement came to exist, and over what period of time. Or, perhaps, both your land and the neighbor's land were once owned by the same person?
3. And once these questions are answers, a lawyer can discuss whether the easement can be modified by agreement. This is very possible, and is done all the time. But when neighbors don't get a long, easements often become weapons in a larger war.
And if I can't help you, I will refer you to some very good lawyers in Frederick.