Breaking and maintaining a set-point

[Blackbirdblue30]

Advice needed. My weight has set at 125 pounds for some years now. That's about ten pounds heavier than I'm happy with. When I was a bit younger it had settled at 112 which I was happy with, but a few years of relationship put the extra bulk of a stone on.
However, a recent stress illness has taken me down to 118 quite suddenly. I really really want to stay here. How do I make my body accept this as its new set point?

Cut out a few sugary things and weigh yourself every morning?

It's like a day or two of normal healthy eating will put it all back on. This has happened before. Yet I can eat the same and more at 125 and stay there.

I don't have a sweet tooth and don't eat much sugar at all...