So, I have been reading up on adaptive thermogenesis - metabolism dropping as you lose weight. The figures I have seen widely quoted are a drop of under 10%. but it looks like some studies are finding drops of more like 20-25%:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3673773/
My own anecdotal experience doesn't seem to bear this out though. I have lost over 50 pounds and currently eat about 500 calories less per day than predicted burn. I lose about a pound a week, which makes sense if you do the math 7X500=3500 and a deficit of 3500 will cause a pound of fat loss (conversely, overeating by 3500 calories will cause a 1 pound gain in fat). If I had a significant drop in metabolism, I would be losing weight more slowly. When I gained back the weight I lost in the past, it was bingeing and generally overeating that did it.

I still believe I can make the lifestyle changes to keep it off.