It's always the same, eat less calories and/or burn more. For about every 7000 calories that you burn more than you consume, you'll lose about 1kg. Your results so far almost perfectly match those numbers:
15 pounds = 7,5kg = 7,5 * 7000 calories = 52500 calories
3 months = 90 days
52500 / 90 = 583 calories per day
That's very close to the 2000-1500=500 calories that you said you are currently eating less per day. If you want to loose weight even faster, eat even less or burn more calories.
I'd personally focus on eating less though. In my opinion it's much easier not to eat 100 calories than to burn 100 calories, especially for people who usually don't do much sports.
But why do you want to loose weight faster anyway? 15 pounds in 3 months sounds like a very good strategy to me. As far as I know, losing weight very fast usually comes with certain disadvantages, and reducing one's weight slowly but constantly is considered the best way anyway.
I recently lost about 10kg of weight at a similar speed and found this speed to be a very nice compromise: Less calories, but not so much less that I was actually feeling hunger. I switched to diet drinks, stopped eating shit too late in the evenings, and TRIED not to eat when I was not clearly feeling hunger. It was surprisingly easy, I didn't even think of me as making a diet.
Oh, and one more tiny tip from me: Don't weight yourself too often. I always laugh at my girlfriend when she once again tells me enthusiastically that she's lost two pounds in just one day even though she did nothing special, or when she is sadly teling me that she gained a pound since yesterday even though she ate less than usual. Real weight loss/gain doesn't work that fast, she lost the 2 pounds probably by simply going to the toilet...







