One of the weirdest thing about being a 'secondary' I've only lately realized that it doesn't really ease insecurity all that much even if you are somebody else's primary. It may and does increase your general happiness with your (love)life, but again, each relationship is unique.
I used to think and advice people who we're having second thoughts about being somebody's secondary that getting a primary of your own would help. But ultimately, you want validation from a specific person, not validation in general as to your general desirablity/dateablity. It doesn't help all that much if all the world thinks you're really hot stuff - you always want that specific person to think so, too