Sustaining Faith

Lately I have been struggling to feel any spark of faith. I recall strong moments in my life where I know I was touched by God, and felt the warmth of His love, but still the doubt and darkness has cast a pallor over my heart and spirituality. I promised that I would remain faithful and true to the word of God, but that promise seems far gone and forgotten. How can I still believe or restore the sure knowledge that I did have? How do I know what I thought I knew is true? Faith ebbs and flows, but how do we get back to the straight and narrow course? I truly am fumbling in faith right now.