JR here's a few pics of my old share horse who was also a TB mare...
When I first knew her it was winter and she had been out of work, and she was a scrawny thing with an upside-down neck and no muscle to speak of -
After a few months of hacking, she was looking better but still had a bit of rounding up to do (note the shape of her hindquarters!) -
By the spring she was looking much better and had put on some good condition -
There was no "secret" to improving Kizzy's condition. She didn't go on a special diet with expensive supplements. She didn't get lunged in draw reins or do any work in an arena. What she did do was gain condition by eating a lot of hay throughout the winter and doing a lot of hacking, including long trots up gentle hills - that is it. No secret involved!
TBs do tend to be on the lean side and in my experience they also change condition a lot throughout the seasons, maybe even more so than other breeds. I don't think Daphi looks that bad, nothing that can't be improved with regular work and plenty of food.
From the way you get treated by some people on here, you'd think we were only fit to be horse owners if we had lots of experience and never made mistakes or had difficulties. The truth is that horse owners are NOT superheroes, for most of them having their horse is a continuous learning curve that never really stops curving. In the "old days" it was common for people with little or no horsy experience to buy their own horse or pony and just "muddle along", making their mistakes and learning as they went. Apparently these days that's unacceptable
Keep on keeping on, as they say, you're doing great