Salvation is not a detached gift of God in some gracious and miraculous way bestowed upon man. Salvation is Christ, and to experience salvation is to experience Christ. It is not the experience of something, but of someone.
So, when I doubt my salvation, I am not simply doubting a gift from Jesus, but Jesus Himself! When I say I am doubtful of my salvation, I am saying I am doubtful of Jesus.
An application is this, that I must look unto Jesus again and again and see how sure He is, how real He is, how true He is, and must not seek something in me.