In spite of what people have told you your entire life, you aren't strong. Deep down you have always known you are weak. Easily confused, easily discouraged, easily distracted... you've always been ashamed of those weaknesses. The flaws people seem to overlook in their kindness, the ones you can never forget.
And that is how you came to understand your purpose. Even though you can't be strong, you can at least be useful to someone who is. Someone with vision. Someone who can take your brokenness and give it meaning. Devote yourself to service and perhaps you can finally be of use.