I've got little sympathy I'm afraid. Did it ever occur to you that now you're 20, your parents might want a holiday to themselves next time they go away? Or that you should probably learn to grow up a little?
When things go wrong and you're on your own, that's the best way to learn how to adapt. I've got a friend who's been babied by his parents his whole life and had never spent more than a day away from them. It was crap - on the plane, he wouldn't sit in the aisle "just in case", wouldn't sit in the middle "just in case", wouldn't spend any time at the restaurant in departures lounge "just in case", and once we were there, we only left the hotel once - he wouldn't go to the beach "just in case", he wouldn't go out on the strip "just in case"..... anyway you get my point. I'm not surprised actually - he hasn't been allowed to ride a bicycle ever since he grazed his knee when he was 12. He's 21 now, not much has changed.
What I'm saying is that if you're that scared to go away without your parents, you should take the plunge, and try to make the most of it - take life as it comes and roll with it. If things go pear shaped, just deal with it and adapt. Whatever you do, don't just curl up into a ball and wait it out, because you'll be wasting your time and money. And if that means if you go with a friend and they end up having to look after you, it'll be wasting their time and money too.
Why not try a solo's holiday? Going away without your parents doesn't mean you have to go and stay in solitude in a deserted shepherd's hut in the hills waiting to get bummed.