Someone practiced in the art of lying is often very believable. With no knowledge of their history, it is difficult to be certain of who they are. This would be an area to assert yourself in the future. Communication is the life blood of any relationship you want to last and grow. Even this will not insure you against the person who sets out to deceive.
Time is a second critical factor.
You mention that your relationship lasted for a six-month period. It is difficult to maintain a charade over a long period of time without an occasional slip. Taking time to really know a person is important, especially when you have met over the Internet, etc. You indicate that you are aware of how you were mislead and how badly your boundaries were invaded. It appears you are using this experience for growth.