You might want to direct your eyes upward, to the tags for this question – specifically "Social Proof" and "Acceptance".
For social proof, you can validate yourself with someone else by showing that you have value to, and relationships with others that they might want to meet/learn from/exploit/etc. This is really really easy to do as a practice – get to know everyone. Fill your days with people, old and new. Just check in for the hell of it. Ask for help. Offer your own assistance. Just be. Remove your own expectations for desired results, and pretty soon all your validation worries will be long gone.
It's also important that you learn to accept yourself, and more accurately that you do so while refusing to be resigned to being just what you are now. There's actually a pretty good discussion of this principle on the question: