If you feel discouraged about lack of progress in Torah study or spiritual growth, look back a few years and see how much you have grown from when you began. (Pachad Yitzchok, Igros Uksovim, p.218)

This experiential proof will supply you with an indisputable refutation to the premise that you cannot grow. Since you already have progressed, you have a good basis for believing that you can continue to improve.