After he and his wife Erica separated following a row over a supposed stolen kiss at a party, the former Household Cavalry sergeant used his Facebook page to tell friends that Aug 31 was the "worst day of my life".
He added: "Sadly have split with Erica, I am absolutely distraught .... still love her very much and would give anything to turn the clock back and try to make things different."
Friends were shocked, replying that he and his wife "seemed to be having a ball at the weekend", and urging him to keep his "chin up". One friend told him his "squaddie buddies will be there for you if you need help".
They perhaps believed that Pedersen would be able to get over the crisis by drawing on the sort of mental strength that had helped him recover from the Hyde Park bombing 30 years ago, when he and his horse Sefton were caught up in a day that claimed more casualties than any other IRA attack on mainland Britain.
On July 20, 1982, Pedersen was taking part in the Changing of the Guard procession when a remote-controlled bomb was detonated as the Blues and Royals rode past, claiming the lives of four of his comrades as well as seven of their horses.
Later the same day seven bandsmen of the Royal Green Jackets were killed when a bomb exploded underneath them at a bandstand in Regent's Park.
More than 50 people were injured in the two attacks and pictures of the horses' corpses lying among the debris became one of the enduring images of the Troubles. But out of the carnage emerged a symbol of hope in the form of Sefton. It suffered 34 injuries, including deep shrapnel wounds and a severed artery, and underwent an eight-hour operation, which was a record for equine surgery at the time.
The black gelding was given a 50/50 chance of survival, but recovered sufficiently to return to military service for a further two years.
Sefton and Pedersen became national celebrities, with appearances on Blue Peter and other television programmes, and Sefton was named Horse of the Year in 1982, when Pedersen rode him at the Horse of the Year Show, receiving an emotional standing ovation.
Pedersen, who was 21 at the time of the Hyde Park bombing, escaped serious injury and the following year he married his first wife, Susan Day, with whom he had a daughter, Laura, now aged 25.
Sefton was retired to a rest home, where it died in 1993, while Pedersen left the Army and by 2001 had started a career as a lorry driver.
His first marriage ended in divorce and in 2002 he married his second wife, Erica Arundale, a driving instructor, in St Austell, Cornwall, and settled in Ashford, Middlesex, where they ran a haulage company called Highroad Logistics.
The couple had two children, Ben and Freya, and neighbours knew Pedersen as a doting father and a "lovely man" who would always help out if anyone had a problem.
But he became bitter after separating from his wife, writing on Facebook: "Why is it that when you ask someone who is hiding something, why they acted as they did they maintain they were 'drunk', yet when something dreadful happens they maintain they weren't and try to assassinate your good name instead?"
He also ended one message by saying: "Sorry everyone."
On Sunday he took his children to visit their grandfather in Andover, Hants, and was due to take them back to his estranged wife that evening.
But instead he drove his Saab convertible to a country lane near the village of Newton Stacey, where he stabbed both of his children to death before taking his own life.