I'm 21 and about to go solo for 2 months in europe and incredibly, my mum doesn't have a problem with it. You know why? I just got back from Vietnam, Cambodia and Laos alone (while she worried constantly). A baptism of fire works well.
At college we used to call it the "pregnancy technique". Goes something like this:
"hi mum. guess what? i'm pregnant..."
"just kidding... [insert lesser bad news here]"
Sorry, shouldn't be flippant about your dilemma. But I think it may be partly just a matter of them getting used to the idea. Be strong!