you need to withdraw. It's the only appropriate thing. DO NOT SANDBAG TO PLAY, just be thankful that you are growing in chess beyond your expectations.
Rating exceeds tournament stipulations
If you met the qualifications for the tournament when you joined, then in my opinion, you have an obligation to complete the tournament.

I agree that there should be no need to withdraw. The only time I would see a need for that if a new player rated 1200 entered and then had his rating zoom up to a high level. In one of my tournaments the biggest upset was by a new player with a 1200 rating beating a 1600 player. By the end of the tournament, the 1200 player was over 2000.
I joined a tournament that was for 1200-1600 rated players. I honestly did not think that I would excell past 1600 hundered when I joined. Of course the tournament takes some time and now I'm rated a little over 1700. should I withdraw? or am I still close enough to the original rating? and is there a point when a person should withdraw? for instances one player in the 1200-1600 tournament has a a rating over 1900! I am considering withdrawing and would like to hear what others think about this.