I have to agree with the opinions of others who say that honesty is the best policy. However, since a relationship with your family is at stake, you can't be completely honest with her. It's a terrible situation to be in. Listen, my dear, you are very young, so you may not know that most people must meet several people before they find "the one". Your lady friend being "the one" for you is like winning the lottery...the odds are that great.
If possible, cut out the romantic stuff with her for now and work on your fade. Just tell her that you both are too young to be dating, because according to WT rules, dating should only be done with a view to marriage. Make sure that questions about the WT that she must answer are included in every conversation you have. Let her do the researching and talking, so nothing can come back on you if she decides to stay in the cult.
I know this is very tough, but please don't make the same mistakes that many of us have made. If you stay in the cult long enough for you two to be caught dating, then much more pressure will be put on her to get baptized and then for the two of you to get married. Then things will be much tougher than they are now.