It's hard to say, but I would say that one difference is that confidence is more like something that you have inside you that radiates outward. Self-assurance is more like something that is coming inside to help fill up a lack of confidence.

Self-assurance is a confidence or belief in your own abilities or character. Confidence can be a belief in yourself or something else. eg: you can be confident that your pen will work. Or confident that your friend will help you. If you meant "what is the difference between self-assurance and self-confidence?" then yes, they are very similar. My take on it is that self-confidence is a feeling you will get if you are self-assured. But you can probably use them interchangeably.

I can't think of any real differences off the top of my head. maybe the latter is more frequently used....