Get your balls back from your girlfriend and it will become harder for her to pull your pants down. Also try one of these:
Leather riding crop, 1.5" tip. Makes a real nice slap & good for softer or more aggressive play
Quality Real Leather Hand Made Braided 21 Inches Long Riding Crop/whip. https://www.amazon.com/dp/6042739026/ref=cm_sw_r_cp_api_.5PAybEMKZNYM