I don't mind answering your questions
The pleasure is all mine.
I guess you're right in doubting your strength in Economics. I myself am very teacher dependent and if the teacher isn't good, I find it hard to sustain. However there are many resources on this website as well as others on the internet. If you don't put all your efforts in the teacher and read from several resources and solve past-papers, you will gain good marks.
Neither field is bad, but as far as I know ACCA is tougher than MBA. :-/ MBA opens your career prospects very wide from advertising to brand management to human resource, and is a very important part of the world's society. ACCA is tougher and requires much more effort than MBA, but once you emerge successful from the test you are able to earn big bucks.
So yes, you should consult your parents and well-wishers and take a chance. Remember: even if you change your mind after your A-Levels have started, with Economics and Maths you can go into both fields.