"Do you not know that those who run in a race all run, but only one receives the prize? Run in such a way that you may win." I Cor 9:24
In this world, persisting toward a goal requires either a want or a will.
When you want something, when you act out of desire, you depend upon feelings of motivation, interest, or excitement to carry you past problems to your goal. You don't need will.
When you will something, when you act out of duty, commitment, loyalty, or integrity, you depend only upon your power to act. You don't need motivation. You persist because it is the right thing to do, or because of the consequences of not doing it, or simply because it needs doing. You call upon the sleeping giant within you, the power of your will.
Motivation is a welcome wind at your back. But winds are changeable and fickle - motivation comes and goes. Will is a faithful friend, at your side in both the sunlit days and dark nights of the soul, speaking quietly of what is important and good.
You can depend upon your will, because it comes from within you.
So sail the winds of motivation when they blow, but in the calm and dry spells, rely on your will to carry you through.