While it can be difficult to figure out how best to sell yourself to a potential employer, the process becomes a little easier once you've determined your ultimate selling point. It doesn't have to be something that others have commended you for in the past; rather, figure out what makes you most proud of yourself as a professional.
From there, branch this pride point out as far as you can. What has it meant for your success? What has it taught you? Why is it relevant to the position for which you're applying? Keep adding new items and keep answering the above questions. Go on until you're sick of writing about yourself. Then take a break, put your notebook down, and look it over after a few hours have passed.
With a fresher mind, pick and choose the items that you feel say the most about you while best advertising your suitability. Keep an eye open for unique traits that you express better than most people. You won't want to flood your description with these traits, but one or two should do you well.
Finally, rearrange what you have until you feel confident in your description. Don't focus too much on what you think the interviewer will want to hear. Instead, put together something that allows you to communicate why you are a perfect fit for the job.