hey miela - You're not a failure, OK? (let's just get that straight right away!)
Everyone has those weak spots and sometimes we find out the hard way what they are. The times when I feel the most tempted are when I'm with a friend who's drinking or a small group of friends. So I can really relate - now you just have to think ahead of time so you have a plan when those situations come up.
You may be at day 1 again, but you still have the success of 3 weeks to be proud of!
Hang in there - one day at a time!!