I really had this problem as well, not know when it is done. I sort of still do now that I have started my Singularity series as I keep adding to them.

This is why I do drawings daily and call them done, even if they are a scriblle with a touch of watercolour, as it gives me that sense of 'done' so I can still work on larger more involved pieces without feeling like I'm wasting time or feeling guilty :)

I love to see your gardens in black and white it is such an interesting approach to all the detail and fantasy of the piece.