First, learn to forget. Forget that you were ever a master. Forget everything you have ever known. Nothing is wrong with your cherished instrument, and nothing is wrong with you. It is simply something different from what you have known before. To know it, you must forget the rest. Forget.
Now, learn it. Learn it as a child learns language, or movement. Learn from the beginning, from the places so novice that you have long since forgotten they exist. Let those places be your new home for a while. Let your prize teach you. Ask it to teach you. Learn how to ask it.
Easy, right? If not, at least it is a path of hope, superior to all other alternatives. A discovery.