You should be pleased. Your success has a good visual of three large stones shed from your body and left behind in a garden.
It is great to have a goal to shoot for, but what will you do if you "only" lose 8 stone? Every stone lost puts you on the road to health improvements. You can certainly meet your weight goal but setting a fixed date for when that happens may not be encouraging for you along the way. Rather, how is your blood sugar? Have you been able to get off medications? From your introduction, there are more important goals than a number on the scale next May.