Haha, I know your pain.
Here's how things work. I'm not sure about the fitting measurements, but from what you have right now and how it fits and doesn't fit, we can figure out what you will most likely need.
Bra's have "sister sizes". That means there are always two different size values that are very similar in outcome... so for example the last time my bras got too big I had a 34DDD.
The sister size for that which is almost the same is a 36DD (wearing the 34 on the smallest and the 36 on the widest setting). I had lost a band and also a cup size so now I'm wearing a 32DDD.
Because when you go down one band size, the cup that comes with that, even if it's the same 'letters' actually changes size too, it gets smaller.
So if you're saying the cups still fit pretty good, but your band is too wide, you will want to go DOWN one band size and then to keep the cups the same UP one cup size.
Makes sense?
So in your case that would be (assuming you lost one band size - that would look like you're on the smallest setting right now and things are kinda getting loose there) a 42G worn probably on the widest or medium setting for the band.
If you're not sure about the cups, do try a 42DDD also.
If you're down 2 band sizes (so you're on the smallest setting right now and still have significant room), but want to keep the same cup size as you have right now you'd be a 40H on the widest setting. If you'd like to try one cup size down with that, also try a 40G on the widest setting.
I know this is confusing but I hope that helps!
Let me know if you still need help