There is hope I promise.
I would see your GP, get your hormone profile checked again. I'm assuming you know Day 21 should actually be called 7 DPO. Get your thyroid, clotting profile, ESR, CRP tested. Even if you're slightly hypothyroid it can make a difference and you would benefit from a small dose of thyroxine. Again with clotting profile, even if borderline, you might benefit from baby aspirin. ESR/CRP show inflammation which can cause issues, there's a white blood cell too, I think leukocytes that indicate possible autoimmune issues.
Ask your GP if they would be ok with you trying progesterone support should you get pregnant again. When pregnant again, ask them to check HCG & progesterone levels. As it plays a part in pregnancy.
If your prolactin is raised, they'll likely retest, then if still high refer you for an MRI to check you don't have a non cancerous tumour causing high levels. They can treat high prolactin with tablets.
For £150 you can get a sample of your womb checked for NK cells. DM me for more info. This if it's an issue is treated with low dose steroids.
You can take B Complex vitamins after the tests, as the B Complex helps your hormones, particularly B6 & B12. There's a variety of vitamins you can take, a lot of it is covered by pregnacares specific conception supplement.
It's scary, it's heart breaking, but please don't give up hope. There's so much research taken place that they know so much more now than a decade ago. Hopefully it's something straightforward and with treatment you'll be fine.
I've had a lot of losses, I also have 2 DC, I studied miscarriages, causes, etc. I also co-operated with studies, so I'm aware of things most gynaecologists don't consider.
Any questions I'm more than happy to help. I would also ask for a mid cycle scan just to check your womb and follicles etc. Your GP can do this, but you should also be sent to a specialist, I would research for a specialist with an interest in recurrent miscarriage.