How come I’ve gained back 20/50 pounds I lost when eating under 2000 cal?

why did I gain back 20/50 pounds I lost if I was still eating under my alleged TDEE?

hi. I'm a 19 year old female, 5'11. 2 years ago I decided to change my life and lose 50 pounds. My methods were not very healthy, I ate between 1000-1500 (sometimes netting down to 700) calories for one year, I focused on cardio, I did not lift, I would go to bed hungry, and I under-fueled to the point where I lost my period for 7 months. When I got to 164 a year later (started at 214), I was happy, but not satisfied. I still saw love handles and belly flab. I was skinny fat. So I decided I would keep going, but little did I know I would start gaining weight.

I started strength training. I became a camp counselor, averaging 15k steps a day and eating 1500 calories, yet gained more weight. I blamed the weight gain on the strength training. My Mom got concerned my period wasn't coming back, so I stopped strength training, thinking it was adding to the problem. Then I went back to school at 169 and by January I was 175. I still tracked, and I was definitely under 2000 calories, which was my alleged TDEE. I tried cutting down to 1200 again, and got down to 168 by December, but soon yo-yoed back to the high 170s. I know you're thinking I just didn't track and fell into my old habits. But after eating so little for a year, by the second year, my stomach capacity was so low. Yeah, maybe on Christmas I ate some calories dense foods and ate 2000 calories. But other than that, I ate the same foods every day (low carb bread, powdered peanut butter, egg whites, protein bars). Even my family was concerned, saying I needed more food. I'm not a newbie- I know you have to recalculate your TDEE every 10 pounds once you lose weight, I know you have to weigh your food, etc. I'm not saying I'm in "starvation mode," but I'm saying there's something that's not adding up. I should include that during this time I included one week long diet break to try and "restore my metabolism," but I ended up keeping the so-called "water weight" I had gained from it, even when I went back down to "normal" 1400 calorie eating. Could it just be a very suppressed TDEE (low bmr due to lost muscle mass, lower TEF, and less NEAT from being constantly exhausted?)

As I started to gain more weight, I got my period back. I thought I would be happy about that. I wasn't. Then by March I was at 178. I decided enough was enough, at this point I had been in a caloric "deficit" for 1.5 years, I maybe thought my metabolism was suppressed and I needed to reverse diet. Well, I did, for 7 weeks. I increased from 1400-1900, about 100 per week and I held if weight gain got too high. I started strength training again. I gained only 3 pounds in the first four weeks, then the fifth week I gained 4 pounds, which I assumed was due to period, but it still stayed there. So I was at 185 at the end of my reverse, completely mortified. I ended it right there and went back on a cut, figuring that much of the weight gain was glycogen that would quickly go away once I ate less.

Now I'm on my second cut, eating 1400-1500 calories, feeling very hungry, and not losing weight.It's been an entire month, and actually, I've gained 2 pounds (slowly increasing every week). And my adaptive TDEE calculator says my TDEE is around 1400 calories, which seems very low for someone my age, height, and weight (is this wrong, or could this be why I gained the weight?). And I'm sitting here wondering, what happened? What did I do wrong? Why is this happening? Any advice would be helpful. I'm desperate. It sucks to blow back up. It's so hard to eat this little. I just cannot do it anymore. Thank you for your time.

BTW took an at home thyroid test with Everlywell and all my levels are normal apparently.