Hi Maisy, I'm not sure I'd go so far as to use the term 'expert'!
Your diet may serve you much better if you add in more fat and protein, especially during the first half of the day. It's amazingly common for people to exist on carbs, often also sugar and caffeine and very little else all day and then eat in a more balanced meal in the evening and we can feel fine on that for ages, years even, but eventually it catches up with us somehow, whether low mood, energy, problems with skin, hair, headaches, binge eating etc. There may well be issues with your hormones that need medical help but tweaking your diet a bit may help and has to be worth a try 
. Start by not skipping breakfast!
Do you like eggs? It would be great if you could have some eggs (any way you like) with buttered toast in the morning. Or maybe some full fat Greek yoghurt and berries and some raw nuts, or have nut butter instead of marmite on your toast. Have a few raw nuts with your mid morning fruit. At lunch, could you choose soup with lentils, chickpeas, beans etc? Some toasted seeds (I love the chilli ones) on the top and maybe a side salad with some spinach, beetroot and feta, or avocado, tomato and cheese? Could you make your chocolate dark (if not already)? Try and pick low GI carbs too, brown rice or pasta or even better, whole foods like the lentils you already have, or beans, sweet potato, quinoa etc.
Cutting out refined sugar can help a lot with pms, I've seen people experience amazing changes, but do what you can with that whilst making the changes above and see how you get on. Hopefully at least your mood/energy will be better such that you have the motivation to cut sugar more strictly if you want to try that.
Give that a go for a week or two and see if it makes any difference.