Keep on stretching. My sister is also 15 and she has never been able to do the splits until now because of stretching. It is good to go online and look up stretches but some that are very good are: -sit and keep your legs as far apart as you can and have someone push your legs further with their feet. Only go as far as you can but far enough that you can feel a healthy pull. -kneel on one knee and put it against a wall. Keep your other legs strait in front of you and stretch as far down to the floor as you can. -go in the splits position as far down as you can go and keep your hands at your side sustaining you up from the floor. Slowly start sliding down and going further down to the floor and stay in a position where you can feel a stretch as far as you can go as long as you can hold it.
Make sure to stretch EVERY DAY. Hope this helps! :)