It's very difficult to change, despite knowing the consequences. Often we don't change until they've actually happened - for some people, this means death.
I'm very lucky in the sense that I knew I had a drink problem before I even considered/was of legal age to drive. For this reason, i'm never getting a car, as I can't trust myself.
All I can say is, I hope you find the strength to succeed and stay sober now. Life isn't over - far from it. Your employer may be understanding - see this as an opportunity to change, but make sure you mean it!