I can imagine the frustration of trying to talk about the issue rationally to your parents and their refusal to accept your side.
I think you need to possibly think up an alternative startegy to communicate your feelings to them.
Maybe try discussing the issue with a teacher or adult friend of theirs who you know who could then put your views across to them - this may make them understand more.
A thing to remember is that in so far as you may feel bitter towards them and their 'ideas' for you, they are probably acting like this as they believe that you are a reflection on them.
Their definition of 'failure' is prehaps none of their children attending university.
I would imagine that it is easy for you to enter heated arguments with them, but try to illustrate to them your true emotional feelings which may get through to them - however the second you start to argue with them they will immeadiately close up and not see anything from your point of view.
Good luck and take care.
By the way where abouts in the channel islands are you from???