I think I'm developing an eating disorder

So growing up I could eat whatever I wanted and would gain weight, I was always naturally thin. Up until I was 21 I didn't give af. Before I went to University I was 10 stone/140lbs. I started taking drugs in my first yearof uni when I was 18 and over the course of a few months I'd lost over a stone. I didn't even notice, nor even cared. I didn't care about looking thin. I stopped doing drugs and started eating, a lot. As I usually did. I gained back the pounds but somehow managed to maintain 10 stone 1 for a year. It wasn't until a couple years later at 21 when it caught up with me. I stepped on the scales and found that I'd gained 13lbs. I know it's not a lot, but I'd never gained weight before and this was new. My father has been overweight for as long as I remember, he was a skinny guy in his 20's but the over eating finally caught up with him in his 30's and now he's constantly in between dieting and overeating. People always joked with me that I need to watch what I ate because I will start to gain weight eventually.

I immediately went on a diet but I knew nothing about nutrition. I'd eat massive bowls of granola with full fat yogurt thinking it was healthy. I'd eat tins of baked beans cause I thought they were healthy. I started out by trying to eat 1500 cals per day but it somehow has escalated to the point where I'm restricting to 500-800 cals per day. I keep lowering my goal weight as well so it's now an underweight bmi. I initally just wanted to lose 10lbs but now it's way more. My height is 5'9, I started out at 153lbs and now I'm around 132lbs. My goal weight is 125lbs.

It's been like this since March. The problem is it comes as a phase. I'll be really into weight loss for a few weeks and then suddenly stop caring as much so start eating normally again, so I gain any weight I lose back and then it's 'omg how could I let myself do that, back to restricting and I WILL be skinny' I don't even want to be stick thin! That's the scary part, I just can't get this thing in my head to go away that tells me I need to be thinner. It's like as soon as I lost a bit of weight, it's like I need to lose MORE, like its a competition with myself to see how much weight I can actually lose. I'm always thinking about food, about weight loss. It's so irritating and my biggest fear is that I'll never be satisfied with it and I'll keep dropping my goal weight until I'm one of those stick thin girls that everyone knows is anorexic. I don't want to look like that. It's acting like any other OCD obsession, this constant cycle in my head where I can't stop thinking about it.