How to thin my waist?

Hi, so I am fourteen years old and just recently got my period. I have never been happy with the way my stomach looks. I am pretty skinny, but my stomach just isn't toned or small-- not BIG but I am uncomfortable in a bikini and in tight shirts. I know that during puberty, girls hips widen and waists thin, and I feel as though my hips widened, but my waist is still big! there is no curve at my waist... my whole mid section is just one straight line. I'm not a very athletic girl, I horseback ride, but don't get too much cardio. i HATE running, cant stand it. I try to do ab exercises each night, but it doesn't seem to be doing anything. My mom always tells me that you can't really control how you body looks at this age, but there must be SOMETHING i can do. Please help!