Sorry, but I don't have much advice for either of you. :-/ I went through the same problem you're having five months ago. It didn't turn out very well lol. I went about getting my passport and visa and flight booked before I told them . . . having physical evidence, some of it non-refundable, that I really was moving helped me keep my resolve. I told them only 2 weeks in advance. I was stuck as well, telling them sooner or later, but I opted for later because, guilty as I felt about it, I didn't think I could long put up with the questions and the prying and the crying that I knew I would get. I set a date for when I was going to tell them, practiced saying what I would say in a NOT shaky and nervous voice, imagined their responses and tried to think up things to respond with. I do have to admit that the planning and the practice helped when it came right down to it. If I had been unprepared, I probably would have broken and they would have had good evidence that I probably wasn't old enough or strong enough to do this on my own. I worried about it so much and was soooooo terrified that for a good three days before I broke the news I spent a lot of time at the toilet throwing up.
My parents have never traveled across the pond, and even though they are fairly wealthy and intelligent people, they have a shockingly narrow view of the world. My mom's definition of a foreign country, English speaking or not, is a place one goes if one wants to be mugged and murdered within one week. Knowing that I would get really crazy reactions like that, I hinted around for a few weeks about the good points of living in a place like the UK, or mentioned how much I wanted to travel to Europe, something they have no interest in doing. This helped set up my reasons for leaving. When they asked over and over why I wanted to move, I could say, "For months you've known how much I wanted to . . ." (it wasn't a few months, but most people don't have memories good enough to realise that).
Trisha, I completely understand what you went through before that move you took. When what I really needed/wanted was my parents' support and help, all I got was their complaints and their questioning of my ability to do what I wanted to do. They never shut up about it, and it got to the point where I didn't even want to be seen off at the airport I was so sick of it! Even though I was dreadfully nervous, the way I slumped with relief when I sat down on the airplane was an indication of how rough it had been to go through their interrogations.
I think that because you are an adult, Trish, you could have an easier time with this than I did, simply because your parents don't really have the "rule" over you quite the way that they used to. You can make them understand that what you are trying to do is very stressful and you don't need them making it any worse. Let them vent and fume for a few days, but once it starts getting excessive, lay down some rules. Let them know that you've heard their questions, their fears, and what they think will go wrong, but that you've made up your mind and you want to leave happily. Then, put the pressure on. It sounds horrible, I know, but tell them you won't talk on the phone if they go on about that subject, then stick to it. I think if you said, "Mom, I'm going to hang up if you don't change the subject," will either change the subject very quickly or end the conversation altogether. After a few of these, your parents will get the point and you won't have to deal with the problem much after that.
Hope that was helpful!
Jewel