I've been dwelling on this for a few weeks, and I don't know how I feel about it. I've tried to be objective and I've done some research, but I still have mixed feelings and no real clarification.

A couple of years ago I lost my appetite, I lost 6.5 stones (91lbs) in a few months and since then I've worked really hard to continue to lose weight not just maintain it. I don't get pains from being hungry, nor do I get the more severe starvation pains I used to get.

This is literally the best explanation I can give, (The same one I gave to my T).
I lost my appetite, the weightloss was so dramatic and instant that I made the decision to use my unexpected weightloss as a starting point to continue to lose weight. I avoid certain food groups, I don't eat breakfast, lunch or dinner and usually make cups of tea with one sugar to ensure I don't faint. I have gone 9 days without food (could go longer happily), and only eat because my partner puts a lot of pressure on me if I don't eat. I get defensive when it comes to talking about food, yet I'm preoccupied with weight 99% of my day.
My triggers for detoxing are simple,
1- if my clothes feel tight, I detox.
2- if my sister tells me I've gained weight. you look thicker in the face, fuller. Chubby cheeks really suit you! My sister is always talking about my weight and it's uaully down to her that I spiral.

For me I want something of my own to control, I want to be able to have something all for me, and apparently I've picked food to do this with. I just don't see the point of putting gas in a car when the gas light isn't flashing. if I'm not feeling hungry, or I simply don't want to eat then I feel like I have the right to choose if I eat or not.

This is where my problem lays, I was diagnosed with anorexia, and BDD. I struggled with the idea of being anorexic for a long time because it didn't feel like it fit, the BDD I could see why my T thought that primarily.
To the point, my T now says she believes that my starvation is a form of self harm which I totally and completely disagree with. I don't know what I am, if I am anything at all, all I do know is I'm frustrated and at a loss. Please, if you can offer any words of advice or comfort it would mean a lot to me.