I think that if you continually look outside of yourself to solve the problem of the feelings of insignificance or inadequacy that you have harbored for many years, you are distracting yourself from changing that internal belief. Helping others or finding an equally satisfying purpose is a great way to reinforce and build upon the already held belief that you are indeed significant. However, I am not sure how good external strivings are when it comes to changing those feelings to begin with.
As you stated, these feelings began at a young age and were then reinforced as you grew older until they became a staple in your own self perception. The trick now is to reprogram those untrue beliefs. I am quite certain it is not the design of the Universe for you to have these beliefs. Actually, quite the contrary.
I would recommend a therapist you trust as well as spiritual books that deal with changing your understanding of yourself. Changing faulty beliefs is a process. A couple of Wayne Dyer's earlier books address the issue in very easy to understand terms. They are "Your Erroneous Zones" and "Pulling Your Own Strings". I got alot out of both of them.