BW:- " ...having to pretend all the time is miserable, especially since my parents still expect me to go in service."
That's just terrible.
Would it help to get a weekend job? You would be able to start saving money and you would be able to avoid the minisrty.
You've broken the ice by informing them that you don't intend to be a JW. It would be responsible of you now to embark on becoming self-sufficient.
It's a good first step to freedom and independence. Being proactive will help you cope with your present circumstances.
If your parents have issue with it, explain to them that you are considering your own future. Would be a good way to demonstrate that you are an independent person deserving respect... a point they seem to be struggling with at the moment.
More importantly, you will start setting your own boundaries. This will help empower you.