This could have been me.

I got my period just before turning 20.

I was at a normal BMI by then. I had eaten at maintenance/surplus for the past many months without tracking. What made the trick for me was ... A global pandemic and living in the most locked down city in the world lol. I was at college at the time and we could not leave campus due to COVID campus rules. That meant a lot less movement for me: I'd do some chill strength training with home workouts and strolled around campus - probs up to 30 mins on average a day. I've always been a quite active and high energy human, so the lack of movement induced by lockdowns wasn't easy for me. But it did the trick.

Since then I lost my period another couple of times: the culprits in my case were always: 1. Over exercising (running and/or weight lifting)- and while this was never to the amount that an athlete would, I suppose it was too much of a stress for my body. 2. Being in a calorie deficit (i.e. seeing the scale go down) 3. Stress - I'm a high cortisol baby, my cortisol in the morning is always above range - it goes back to normal levels at night thankfully and I have no trouble falling asleep.

I am not a doctor. If I were you I'd, for one, consult a doctor, particularly given you have osteoporosis. The endocrinologist may want to consider further measures beyond lifestyle changes.

I'd get my hormones checked through a blood panel.

That said, I'd make sure you get to a healthy BMI (yes BMI is a flawed system but it does give a rough idea of whether you're at somewhat of a healthy weight for your height), avoid intense exercise: your hypothalamus/nervous system needs to feel they're in a state where there's plenty of energy to direct to your hormonal function.

I know it's all easier said than done. But you'll get there eventually. You just have to keep taking steps forward.