question about my weight

hi i’m 17 female and my height is 5’9.5 or 176ish cm. my weight is 10stone or 54 kg is fluctuates. when i used to workout for 15 - 40 minutes and eat a piece of fruit and a sweet potato a day i would only lose like 0.5 - 0.40kg a day depending on how long i worked out (i only did that for 10 days) but now i’m not working out just eating 1 meal a day and a snack it’s probably around 500 calories a day, i don’t lose any weight. i only lose weight if my diet is very extreme like 100-200 calories and no sugar or additives or anything, or even if i ‘water fast’, just a fancy word for starving i think. i only did it once for nearly 3 days and lost 0.20-0.70 during the final days i did it which is the most i’ve ever lost. even when i workout and eat normally ish i don’t seems to actually lose any fat just gain muscle so i don’t know what to do. i don’t know if it’s because my calories have been restricted for so long but i don’t know how it works. i want to be 55kg and before you tell me i have an eating disorder i know i’m aware i show signs of having one but i’m dealing with a lot at the moment and i don’t know how else to cope. it’s the only thing that makes me feel like i’m in control and i can be beautiful in my head not social norms per say because i know being thicc is the standard right now. but if anyone’s knows why this is happening and when i don’t lose or gain weight i just say the say unless i take things to the extreme i would really really appreciate it. thank you!!