The Chinese philosopher Lao-tzu once said "A journey of a thousand miles begins with a single step".
True sexual purity is a great achievement in today's world, and for some people it may look very far off. But take that first step today and you will get there in the end. Every time you say no to yourself, no matter how insignificant it seems, you have taken another step. If you fall, you don't go backwards--you are still where you left off last. Just stand up and continue the journey.