A large part of being able to do splits is genetic, and what I mean is certain peoples hips and/or head of the femur bone (the one that goes into the hip) are made in such a way that you can only physically get so much hip movement.

A big part of getting the splits is lengthening your hamstrings and quadracepts and the leg muscles are such a complex muscle group, if you ever saw a human muscle chart in a physiology or anatomy book you will start to see what I am talking about.  It takes lengthening (not stretching) them properly to open them up.  A lot of flexibility and movement in ballet has to do with release and contraction, you have to have train your body with the ability to release your muscles at a moments notice and how to engage one muscle to create the effect, so learning to control your endurance and extensions is required to do the splits and large leg extensions.

If you have been stretching improperly for a long time you may have created scar tissue in your muscle fiber.  This scar tissue will limit your flexibility, depending on how much you have it is difficult to say if you can recover.  Hence when you tear a muscle why you loose a certain level of your original flexibility.

To get past this stage and since you have been stretching regularly it seems.  I would suggest you seek professional help.  Find a Kinesologist of Physical Therapist who works specifically with dancers in your area (if you cant find one that works specifically with dancers easily, check your local professional level dance companies/studios or local universities as some of them have people they work with).  

A person with the training of how the body is built and moves will be able to tell you if you have the proper bone structure to permit the splits, if you dont how far your body will actually go into a split prior to injury and how to open the muscle groups to make your lengthening exercises more efficient and effective.

hope it helps you.

Ron.