You want to get more flexible for doing the splits. Getting the 180 degree line for a jete or a penche does not come naturally to many dancers. So how do they make it look right?
One of the biggest reliefs I had when I went from amateur to professional training was that hips do not have to be square in a derriere (behind you) position. Including doing the splits.
When I got into professional classes at The National Ballet School of Canada, I was elated to find I could open my working hip. The waist, upper back and shoulders had to stay square, but not the hips. I finally and instantly had a professional looking line in arabesque, attitude, etc. When I explained how I had been taught they said "no one can do that!"
Some people will never do the splits due to hip deformity, and should not over work the issue.
Doing the splits depends on overall hyper-mobility. Not only hamstrings and quads need to be extremely flexible, but your postural muscles, the iliopsoas, needs to be very flexible. Hyper-mobility of the joints (meaning longer stretchy ligaments), is an extra blessing for doing the splits, but creates a lot of problems too.
A professional ballet dancer will do whatever it takes to get a good line in a split jete or penche. Those who cannot do the splits perfectly open the hip more, and sometimes slightly bend the leg so that their foot lines up with the hip, and even though the entire leg is not lined up, the illusion of the splits is seen.
The hard and fast rules of classical ballet training are for safety - to prevent dance injuries. Getting the right line allows for accommodations that skilled teachers know how to teach.
Stretch after your ballet exercises when you are warm. Relax your muscles first. Use a rubber ball to knead out the worst tension. Then stretch gently in correctly aligned positions. You will improve your muscle flexibility, and you may end up doing the splits. But if you never do, it is not going to kill a dance career.
Here is a wonderful DVD produced by a dance expert, Deborah Vogel which will help you get more flexible. It is excellent for non-dancers too.