pav,
I am taking my part 3 in architecture this year, but I have gone through a similar experience myself (I even posted here about earning a living from SU etc).
I would probably agree with Alan, and suggest you stick with it for now, but make the most of any opportunities that come your way. Usually people take a year out between parts 1 and 2 (although there were quite a few at my uni who took 2 years out - actually there were alot who just didn't come back). Make the most of this time and get as much experience as you can (both architecture and visualisation if you can), and at the end of it you'll hopefully be in a position to make a more informed decision.
I would suggest you speak to your tutors/careers advisor/professional studies advisor/etc as has already been mentioned it is a common experience and these people should be able to give some valuable advice. What about talking to students who are now taking their part 2? they may have gone through the same and have some useful suggestions.
If you don't mind I would like to ask what it is that you are not enjoying about architecture and why you don't think it is for you?