Noelle Reno, 29, said life had been "incredibly hard" since Young, 51, who is originally from Dundee, was jailed for six months at Pentonville prison for contempt of court arising from a long-running divorce dispute with his estranged wife Michelle.
Ms Reno, who is a fashion designer, admitted the "crazy circus" divorce case – believed to be one of the biggest in legal history – had put pressure on her relationship with Young, but she said she still hopes to marry him when the case is settled.
His jailing came after a High Court judge last month found him to have disobeyed orders to provide financial details to his wife and to be in contempt of court.
Young claimed he lost a £400 million fortune after separating from Michelle in 2006 and, due to his bankruptcy, cannot pay the £27,500-a-month maintenance ordered by a judge. Mrs Young said her ex-husband has "secreted enormous assets".
Since being jailed, Ms Reno says his high-profile friends – who reportedly include Top Shop boss Sir Philip Green, X Factor impresario Simon Cowell, restaurateur Richard Caring and Russian tycoon Boris Berezovsky – have distanced themselves.
"Some people have really taken distance from him. I am not saying they are cutting Scot out of their life, but the less contact they have with Scot the better," she said, singling out entrepreneur Sir Tom Hunter as the one exception.
"The more contact they have – if they help him out with money or are seen with him – [Michelle] starts thinking 'oh that must be because they are hiding all of Scot's billions of dollars'. So it is difficult for him to have high-profile friends right now."
Ms Reno dismissed suggestions the pair live a life of luxury after being spotted at high-end restaurants and bars during their three-and-a-half-year relationship.
She added: "Scot's been completely, totally broke for the duration of my entire time with him," she said, noting she pays the bill when they go out.
"It might look like we are leading an incredibly glamorous life but a lot of it is complimentary. I get lots of free stuff doing what I do."
She also revealed how Young had explained his finances: "He told me exactly what everybody else has been told – that he had an earn-out when he sold his tech company. He put the majority of his money into the Moscow property scene and that went awry.
"Scot was always living a life beyond his means. It is that simple. Like many people, he was never actually living the lifestyle he was [seen to be]. He didn't really actually have that much money."
Ms Reno has shed tears during visits to Pentonville prison and admitted she is now emotionally detached from the legal battle because it has "destroyed so much of my life for so long".
She said: "It puts a lot of pressure on our relationship, for sure. I am getting to an age now where I don't really want to wait. But I am very strong. Whatever happens we are always going to be best friends, even if he never divorced and we did split up."
Ms Reno still hopes the pair will wed and has some simple advice for what Young should do when he leaves prison: "He needs to finish his divorce and get out of bankruptcy."