The 'Inception' star has been clean for 10 years after checking himself into rehab to deal with his alcoholism and crack cocaine use in 2003.
The actor told the Daily Mirror newspaper, "If I had four pints of lager and half a bottle of vodka I could turn this room into an absolute f***ing nightmare in about three minutes. I could destroy everything in my life I have worked so hard for.
"If I stop working they might take it away from me. People will say, 'Tommy you're doing well,' and I say, 'Am I?' In my head I'm still the kid. Does that make sense?"
He continued: "I love what I do, but it's driven by a fear of not being able to do it. It's the same with drinking - if I stop then who am I? What have I got? I have to watch that drive.
"It doesn't matter how well I am doing, I'm only that far away from f***ing it all up."
Tom, 36, has a five-year-old son, Louis, with ex-girlfriend Rachael Speed and is now engaged to 32-year-old actress Charlotte Riley.
Tom - whose marriage to ex-wife Sarah Ward broke down in 2004 - admits he had to lose something he cared deeply about in his life to realize he did have a problem with alcohol and needed to quit drinking.
Tom said: "I did something particularly heinous that allowed me to wake up.
"I had to lose something. Sometimes you have to lose something that is worth more to you than your drinking."