I also look back at photos and wish I was that slim. But a few things come into my mind now I think of it.
In some of these photos I was deeply unhappy, especially teen ones, I had a full blown ED and severe depression. Life was in no way happy, home life was chaotic. But, yes, i look young and slim.
Then another time about 4 years ago when I was starting a business up and it was very stressful, i lost loads of weight and was on the ball and excited about my new venture, busy all the time. In fact too busy to eat. I passed out a few times, once on the stairs going up to the supermarket. So, yes, i was thin but in no way healthy that was sustainable.
I think it's easy to look back and only focus on what you see body wise. When i first see the photo, i only see skinny. But when i look deeper it wasn't great.
You said the stress made the weight drop off. That's not a healthy place to be.
I am much happier now in life. I just need to find balance in eating and self image. I'm not sure I would swap present me for that past me just to be thin again. Hopefully i can find a balance between happy, contented me and a body I'm happy and content with. Maybe it has to be a conscious decision.
My mantra for today will be "I am happy and content with who I am (including my body)"
Maybe then I'll start treating it better with healthy food in healthy quantities and enjoyable movement and exercise.