Hi there
Never done ballet in my life

but have been working on my flexibility and core strength due to lower back injury.
I would guess you need to work on two things:
1) Strengthening buttock and lower back muscles in order to have more power to lift the leg back
Buttock exercises - squats and lunges, or from all-fours, lift and straighten leg behind you (keep rest of body controlled and square at all times).
Lower back - depends on how strong your back is to start with really. A hardcore exercise is to lie on your front, tuck your feet under the sofa edge or similar, and lift your upper body off the floor into this position:
http://www.yogaworkouthq.com/wp-cont...cobra-pose.jpg
and lower back down again. Start off using your arms as support and work up to using your back muscles only.
2) Stretching the hip flexor at the front of the hip
This shows the basic position:
http://www.fitsugar.com/Stretch-Hip-Flexor-Lunge-766938
Important point is to keep your pelvis/bum tucked under.
To stretch further in stages: lower the back leg, grip the back toe and bend the knee to pull the toe towards your bum, also you can stretch backwards with the upper body.
So you should end up being able to do this
http://t3.gstatic.com/images?q=tbn:A...W6DYAJDeyFaMO3
You would probably need to stretch your quadriceps as well.... basically stretch everything

and daily....
Obviously please check with a physiotherapist/yoga teacher/pilates teacher for proper instruction.