Assumption is not a bad thing if your assumption is based on reality. Most assumptions are, however, illusions of what we want to take place. The problem with what just happened is that you assumed the wrong thing and you were betrayed.
There is always an element of shock in this kind of discovery, but in every betrayal there is a promised resurrection. Hold back your response until the shock of this incident subsides and then ask the Lord what your course of action should be. Silence will be your friend and protector until the Lord speaks.
Before this incident took place you only saw this relationship within a limited understanding. Now, with a fuller understanding of this new reality you can offer wholeness and health first to yourself and then to all parties involved. None of this will be lost if you choose to not throw people away in your response to this sorrow. Assume the best – this is your new assumption.