It is not the end of the world. Discard the failure mentality. Your goal is long-term sobriety. Get back on track and continue to plug away. Sometimes a lapse can serve to reinforce you path and goals. You may have an epiphany that using is not fun, not healthy, dreadful, and that you never want to be there again. It can also help you identify certain triggers/emotions that need attention.
“Never confuse a single defeat with a final defeat.” – F. Scott Fitzgerald