Q: I can't straighten both legs and maintain a closed fifth position. I believe it's mostly because one leg is longer than the other one. What can I do?
A: It sounds like you have a true leg-length difference rather than a functional discrepancy. A true leg-length difference is when the length of the bones on one leg are significantly different from the other. A functional discrepancy is when the bones of the two legs are the same length but there is a postural asymmetry that is throwing your alignment off.
Let's try a simple solution first: Stand in first position facing the mirror. Slowly lower into demi-plié. Do you shift to the longer-leg side at the bottom of the plié? Now put something small—roughly one inch in thickness—under your short leg and repeat your demi-plié. Does it look more even? How does it feel? It's not unusual to get enthusiastic about how much better fifth position works when you're even.
Beyond fifth position, it's important to address spinal alignment when dealing with leg-length discrepancies. I've seen many dancers with one long leg that has created a scoliotic response in their spine. When a lift is placed under the short leg, the spine straightens out. It's lovely when you can easily balance how the weight falls through the whole body.
If it isn't a true leg-length difference, further evaluation is needed. Is there muscle tightness in the lower back or hip? Postural patterns that are affecting the spine? Find a good doctor and/or physical therapist to assess your alignment. In the dance world, balancing out the small differences can make a huge difference in your technique.