I have to tentatively disagree with the common pulse of this thread.
1) It doesnt sound like she dislikes being tickled, she is just concerned it will be too much for her- which is very well may be
2) The suggestion that if you love someone who is against something you want in the relationship, the best course of action is to abandon it without further exploration, doesn't hold a lot of water for me.
My advice is to learn the girl and her motivations for saying no. If she hates the idea then I have to concede the point that has been made countless times in this thread- a hard no is a hard no. That said, it doesn't sound like she's there. Other suggestions:
1) As others have said, help her learn to love it. Mixing affection with increased tickling can work, or help her see how sexy you find it and how turned on it makes you.
2) Bargain. I know that sounds clinical, but as someone mentioned- what if she has a ballbusting fetish or something hopefully more survivable. I have entertained girls with unique fetishes because in turn they were willing to entertain mine, though it did little for me and tickling did little for them. Give and take is part of a relationship (caveat: IF she doesn't absolutely hate it), but be prepared to match the sacrifice you are asking her to make.
3) Compromise. There are a lot of steps between playful tickling in the bedroom and the kind of films that producers make. She has already taken a step saying she wants to come around and is trying- offer her a partial step. Tie her hands but not her feet. Blindfold her with no restraints. Let her ease into the idea of helplessness and see how much she can take- it may work like ladder rungs to where you want to be.
In closing, decide what is important to you. If you like the girl and you like tickling, and if none of these methods work, you may have to make a choice as others have said or implied.
Best of luck.
