Recreational ballet is for everyone at any age. However for a profession you are way too late. Real ballet training doesn't start until age 7 or 8 and before that is pre-ballet and doesn't count. So basically you have had no real ballet training. Although gymnastics helps with flexibility the alignment is very different for ballet and many gymnasts have a real problem switching over. Ballet all comes from your turnout and that isn't something that gymnasts use.
You real problem is that at 15 there aren't enough hours in the day to catch up in regards to professional training. Dancers on a track to be professional ballet dancers study in ballet academies taking 20-30 hours of class a week year round at your age and have done so for years. By age 16 their training is complete and they apprentice with a professional ballet company. Some even apprentice as young as 15. It takes 10 years to "make a leg" in ballet. If you were to study intensively at a ballet academy (they don't train girls your age) you would be ready by age 25 . That is the time most ballet dancers start thinking of retiring. If they aren't injured many leave the ballet by their late 20s.
To become a professional ballet dancer, you must have a ballet body. That is way more than just the right height (5'2"-5'7") and weight. Only 2% of the population is born with this body type. It involves high arches with a high instep, great turnout from the hip rotators, long legs, arms and neck, short torso, long stretchy achilles tendons, slightly hyper-extended knees, and tons of facility along with a few other things I have left out.
My advice to you is find a teen or teen/adult beginner ballet class. You don't have to take class with children most likely. There are plenty of beginner classes for older girls. Dance can always be part of your life and there is no reason why you couldn't get quite good at it an even end up eventually dancing en pointe after a few years. There are performance opportunities in small civic amateur ballet groups. Dance can always be part of your life. Who says you have to get paid to do it.
My daughter is a professional dancer. I worked for NYCB (New York City Ballet)