The private thoughts of one of the US's best-known first ladies were sold at an auction in London on 29 March.

The 18 letters exchanged between Jacqueline (Jackie) Kennedy and former UK ambassador to the US David Ormsby-Gore, Lord Harlech, over the course of seven years from 1961-1968 were sold for £100,000 ($124,000, €116,000).

John Fitzgerald Kennedy's assassination in 1963 turned his much-admired wife into a young widow. Ormsby-Gore became a widower four years later when his wife Sylvia died in a car crash.

The diplomat had become a close friend of the Kennedys during his time as Britain's ambassador, from 1961 to 1965. His friendship with Jackie continued to blossom in the years following their partners' death, to the point that Ormsby-Gore decided to propose in February 1968, shortly after the two went on a trip to Cambodia in November 1967.

But Jackie had decided she was going to marry the billionaire Greek shipping magnate Aristotle Onassis, seeking more than love, protection from what she had come to see as an increasingly threatening world.

In her rejection letter, Jackie wrote: "We have known so much & shared & lost so much together – Even if it isn't the way you wish now – I hope that bond of love and pain will never be cut... You are like my beloved beloved brother – and mentor – and the only original spirit I know – as you were to Jack."

Ormsby-Gore's response to the letter was never sent. However, a draft recovered among his correspondence speaks of his heartbreak: "All the pathetic plans I had brought with me for visits to Cyrenaica, holidays near one another and a whole variety of solutions to our marriage problem, including one for a secret marriage this summer – plans which I saw us eagerly discussing, calmly and with complete frankness as we did at the Cape and in Cambodia for the next wonderful ten days – all had become irrelevant trash to be thrown away within a few hours of my landing in New York.

"As for your photograph I weep when I look at it. Why do such agonising things have to happen? Where was the need for it? I have tried for hours and hours to understand your explanation and I suppose I do in a way, without agreeing with it; but what I find unbearable and in a way, dearest Jackie, untrue is that you could come to such a categorical conclusion..."

In another letter sent to her suitor from Onassis's yacht Christina, Jackie tried to further explain her decision: "You and I have shared so many lives and deaths and hopes and pain – we will share them forever and be forever bound together by them... If ever I can find some healing and some comfort – it has to be with someone who is not a part of all my world of past and pain – I can find that now – if the world will let us..."

The letters form part of a stack of papers that were recovered from two boxes at Glyn Cywarch, the Harlech family house, after Lord Harlech's death in 1985. It included personal correspondence from President Kennedy and from British Prime Ministers, Harold Macmillan, Sir Alec Douglas-Home and Harold Wilson.

Auction house Bonhams estimated the archive at £100,000-150,000, but the 10-hour long action raised a total £2.6m for the 531-item lot, the BBC reported.