The tormented dad saw the tragic teens MINUTES before they joined hands and leapt 125ft to their deaths last Sunday.
Now, as one of the last people to see the girls alive, he is plagued by the thought he could have stopped them.
John, of Erskine, Renfrewshire, said: "Over the past week I've been thinking, 'What if I'd stopped? Could things have turned out different?'
"Now every time I drive over the bridge, I think, 'What must have been going through their minds? What was the last thing they were thinking?'
"I don't think I'll ever forget that night . . . ever."
Landscape gardener John, 23, saw the girls as he drove to a customer's house in Bishopton on Sunday. He explained: "At about quarter past eight I spotted them walking on the other side of the road from me, towards Erskine.
"They were walking side by side, chatting. They didn't seem in a hurry or upset. They seemed happy and I didn't think much about it. I thought nothing of it - girls are always out from the school."
John saw the girls again as he made his way back to Erskine at around 8.25pm. He told us: "I saw them at the Erskine roundabout, walking towards the bridge. They still looked happy and were chatting away quite normally.
"I didn't think anything of it. Nobody would have thought, 'Those two girls are going to jump off the bridge'. It didn't cross my mind to phone the school or the police.
"It's quite common for girls from the school to abscond and they're usually picked up very quickly."
Dad-of-two John, who was in care himself as a child, added: "When I heard the news on Monday I was shocked and realised who they were. I can only imagine what they must have been going through to do this.
"I'll never forget seeing their faces that night and, as a dad, my heart goes out to their poor parents."

HOLLYOAKS SUICIDE
A GRIEVING pal of Georgia Rowe claims the teenager was obsessed by a suicide plotline in TV's Hollyoaks.
Troubled character Barry "Newt" Newton was shown on Friday trying to take his own life by leaping into water from a warehouse roof.
And a friend says Georgia repeatedly watched an internet trailer previewing the shocking scenes in the days leading up to her death.
The 14-year-old said: "She was obsessed by this Hollyoaks thing. She loved it and she watched every episode.
"She'd seen the trailer on the internet over and over again. It made it clear how he was going to end it - by jumping off a high ledge.
"Newt had demons inside him too and she connected with that. She was the same. She told me she used to hurt herself and she had scars on her arms. I didn't know whether to believe her."
The pal told how Georgia's room at the Good Shepherd Centre in Bishopton, Renfrewshire, was full of Hollyoaks books and epsiodes of the show on DVD.
In the hit Channel 4 show, Newt - played by Nico Mirallegro - forges a death pact with a friend he makes while on the run from the authorities.
The storyline chillingly mirrors Georgia's bridge suicide on Sunday with her 15-year-old friend Neve Lafferty.
Georgia's pal - who does not stay at the Good Shepherd - said: "Georgia must have got the idea from the programme - especially as it shows a pact with a friend.
"When I heard what had happened I wasn't shocked, it made sense. The story had been everywhere, it was in all the magazines. I know the episode wasn't shown until afterwards, but Georgia knew the storyline and knew it was coming up.
"It's very sad. Georgia and Neve were in a really bad place."
A senior social work source said the shocking discovery of a possible link to the TV show had left welfare bosses in Renfrewshire horrified.
She added: "They can go on about issuing on-screen warnings and publishing helpline numbers, but it doesn't change the fact teenagers should not be exposed to this sort of thing."
Channel 4 last night confirmed they had received a string of complaints over the decision to go ahead with screening Friday's episode.
The broadcaster admitted there were "similarities" but insisted they had "carefully considered the implications" of showing it.
But one leading academic blasted the "immoral and unethical" storylines on some TV shows. Teenage suicide expert Dr Arthur Cassidy said: "TV producers have an ethical and moral responsibility not to contribute to harming young people's mental health. These dramas portray suicide in an odious way and this can have a devastating effect on young people's lives."
Dr Cassidy added: "There is evidence that continuous exposure to teenage suicides in soap operas together with other factors - such as song lyrics and cyber bullying on networking sites - can substantially act as a stimulus in suicidal behaviour."
A spokesman for the Catholic Church in Scotland, who run the Good Shepherd unit, said: "Hollyoaks was certainly a programme the girls watched. But there were no signals that either of these girls showed a particular interest in this storyline."
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At peace now, you pretty little babies, who never had a chance in this messed up world which failed your hopes and aspirations.Goodnight and God bless and rest in peace.
By Dazed and Confused by the 21st Century. Posted October 11 2009 at 1:56 PM.