Often, we are not the best judges of our strengths. We think we are, but a view from the "outside" is often more reflective of reality.
So, after you have developed your list of strengths, ask a friend or former co-worker (more than one, if possible) if those are the strengths they would choose to describe you. Their answers could surprise you, and, probably, will be very helpful.
Ask for examples of when you demonstrated that strength. Then, put together a very short narrative of why something is a strength for you. Have additional proof available.