Legato is not something you can just practice and you get better (although you can try and take a very, very long time to even achieve a bit of what you want). It is entirely a matter of technique. Because it is a matter of technique, playing Mozart, Bach, or any other kind of piece or study will not help. What will help is a teacher, a very good one for your situation, who specifically knows how to teach you the technique of legato.
From my observations of other students and teachers (even ones at universities with doctorates), I have seen that very, very few even understand the sound of legato, let alone the technique. What is considered legato by many is not actually legato at all! It's appalling that there are teachers who teach, who are highly respected, have had wonderful performing careers and 1)can't play legato or 2) if they can, don't have any intellectual understanding about how to teach it to others.
The problem about legato is ignorance about what must mechanically occur in the piano and what your body must do to achieve this action. Find a teacher who at least knows what your body has to do to get this legato effect. Someone who prescribes Czerny, Bach, Mozart, et al knows little to nothing. Look elsewhere. The alternative is to go to a library and search for books about piano playing and look through the table of contents for a chapter on legato. Many books have it but many are inaddequate especially the older ones. Try Sandor's, On Piano Playing, to name one which deals almost exclussively with technique. Then after you have read the chapter(s), apply the intellectual information and turn it into practical information by applying it. This is what I have done because of the poor instruction I have recieved and I was tired of teachers telling me to "practice at least 3 hours a day [and you will get better]". I practice significantly less now with far better results.