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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."

Irish News editor Noel Doran said they had "got it wrong" over flag protests.
Loyalist protests followed Belfast City Council's vote last December to limit the flying of the union flag from city hall.
Some of the demonstrations resulted in violence.
On Saturday, Mr Doran told the conference in Lurgan, County Armagh, what had been opposed had been a "democratic decision" of Belfast City Council.
Mr Doran told about 140 delegates that nationalists and unionists often viewed each other with "mutual incomprehension" even though they had a lot in common.
He said loyalists could learn from organisations like the Gaelic Athletic Association, when it came to issues like community pride and reaching out.
Progressive Unionist Party councillor John Kyle said loyalists had to do "more than" protest; he said they also had to come up with ways out of problems that arose.
He said the party would not give "cover or excuse" to criminality.
Party leader Billy Hutchinson was asked about a recent shooting attack in east Belfast in which a 24-year-old woman suffered injuries to her lower abdomen, hips and knees.
Police have said they are treating the attack at Lord Street Mews as attempted murder.
There has been speculation that the UVF were behind the shooting.
Mr Hutchinson said he condemned it.
"I'm not the leader of the UVF, I'm the leader of the PUP, but I will answer it, it was despicable," he said.
"I said that at the time, irrespective of who was responsible, there's no place for those people in society and they should be removed."
During the conference, delegates also passed a motion supporting equal marriage in Northern Ireland.
The motion was: "This party joins with remainder of UK in enshrining into law equality for all in marriage.'
Meanwhile, party chairman Brian Lacey said he did not recognise the party as it was sometimes portrayed by the media, adding that if it were like that he would not be a member.
He said he was proud to be a party member adding that membership had tripled in the past year.
Rhys Webb says he knew his World Cup was over the moment he was injured in Wales' final warm-up match.
[NEXT_CONCEPT]
A representative of the nationalist community in Northern Ireland has addressed the Progressive Unionist Party's annual conference for the first time.