The material in the seperate parts is really nice, but I feel that formwise, it doesn't really work as a piece - the sections are too similar for me to really experience this as a theme with variations. At the same time, the transitions are a bit too clumsy to really work as a "normal" piece. I'd either try to make it flow better as a regular piece (I experienced the 1st A-section as an intro to A' more than anything), or really, really change things up between the variations.
Hope this doesn't come off as condescending or anything, but have you had any references you've been looking at for the theme and variation form? I find the 2nd mvt of Schuberts Death and the Maiden as well, of course, as well as Bachs lesser known Aria Variata to be good, very cohesive examples.