Oh your poor DD - it’s an horrendous thing.
I was diagnosed in my 30s after peri meno ramped it up.
I was offered the pill and anti depressants which I wasn’t keen on. I started on a really consistent, strict regime of magnesium, D, evening primrose and high dose b6 supplements. No caffeine and no sugar. Regular sleep, exercise and cycle tracking.
After a couple of months of that not only was my cycle like clockwork almost to the minute but the symptoms lessened hugely.
I still had very dark days just before and after my period but I knew to expect them and to work around my moods. It helped to know that if I woke up and felt like the world was ending and it was all pointless or I was in a towering rage over nothing, that it wasn’t real and would pass, like a cold.
You mention progesterone gel - for me progesterone is the driving force of PMDD. My symptoms start and increase in the progesterone stage of my cycle. In hindsight the times I have been on the POP were also some of my worst, mentally.
I’m now on HRT and having to take the progesterone part vaginally as it’s too much of a risk orally.
A gynae I spoke to about it also flagged that some people are just much more sensitive to hormonal fluctuations, which is biological. To the extent that a lot of women who have historically been diagnosed with bipolar actually had PMDD / hormone sensitivity and were ‘cured’ by menopause.
There’s also a high crossover with people with PMDD also being neurodiverse. In my case, at its worst, it would trigger severe anxiety, OCD and I was diagnosed with ADHD as an adult.
Mindfulness can also help but as a teen that probably isn’t appealing!
As a starter get her on the b6 at the highest dose you can find for now and add the rest as you!
Good luck to her.