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The Ospreys scrum-half, 26, went down in agony after his leg was caught in a ruck against Italy and he was taken off the pitch on a stretcher.
Webb has had surgery on his damaged foot but the injury was so serious he will also miss the 2016 Six Nations.
"It hit me straight away, I was an emotional wreck as soon as it happened to a couple of days after," Webb said.
"I still think to myself now: 'How am I here and the boys are playing?' But I'm back driving, back in the gym now.
"The amount of pain I was in I knew straight away that my World Cup was over.
"I'd worked so hard over the last two years, with my goal getting that number nine shirt and obviously another one was playing in the World Cup in this country.
"Seeing that go out the window by one freak accident, I was pretty heart-broken."
Wales' opening wins over Uruguay and England have put them in a strong position to reach the quarter-finals - and Webb says he has been cheered by those victories.
Last season's Pro12 player of the year has now set himself the target of selection for the 2017 British and Irish Lions tour to New Zealand.
"I've picked myself up slowly seeing the boys win like they have done and playing well - it's just looking forward now.
"My long-term goal is British Lions - that's everybody's dream - but I need to set some shorter goals so I'll do them in a couple of weeks maybe."
Webb was back among his Wales team-mates on Wednesday as he and fellow injury absentee Leigh Halfpenny - the full-back ruled out of the World Cup and Six Nations with a knee injury suffered in the same game - presented the match jerseys for Thursday's game against Fiji.
"It was obviously a special occasion," Webb added. "I gave a bit of a speech, which was nerve-wracking, but it was very good, I enjoyed it - I was more nervous presenting the shirts than playing.
"Dan Lydiate and Toby [Taulupe Faletau] are winning their 50th caps so it was a pleasure to present them with their jerseys as well."
Rhys Webb says he knew his World Cup was over the moment he was injured in Wales' final warm-up match.