A close pal said: "She and Wills love each other dearly but Kate has just woken up to what the next few years could have in store.
"She is suddenly realizing the enormity of what could lie ahead if she walks down the aisle with William.
"It's one thing having an on-off relationship with Wills, it's quite another thing becoming Queen."
Kate and William's romance has been rekindled since they originally split in April.
Wills, who is 25, said he wanted to concentrate on his Army career.
Within six weeks, however, the couple attended a ball at Bovington barracks in Dorset, after the prince completed a tank commanders' course.
Then Kate, also 25, was a guest in the royal box at the Concert for Diana in July—a clear sign the relationship was back on track.
And recently, say friends, the relationship has gone from strength to strength.
So much so that Kate has grown increasingly worried about where it all could lead.
One source said: "Kate is concerned that those before her who have married into the royal family have come from an aristocratic background and are from old money."
"She has seen how the royal family washed their hands with Diana—and the princess had a privileged background and came from the right breeding.
"Even Sarah Ferguson's dad was an army man through and through and was part of the polo set. Kate is just from a very normal middle-class family who have done exceptionally well for themselves.
"They don't live on a huge estate or have titles before their names."
Kate and Wills have gone to great lengths to avoid being pictured together in an attempt to ease the pressure on their relationship.
