Hi Rosepetal if you're still having regular periods it's difficult to know what to say. Had you missed any at all and how regular is your natural cycle? if the progesterone is now tied in with your natural cycle then you will be adding to your own progesterone. The PND points to needing more oestrogen so this should help also with PMS. PMS can also involve progesterone intolerance (you would normally have symptoms throughout the second half of the natural menstrual cycle) and progesterone withdrawal (intense PMS symptoms after stopping the progesterone or just before you natural bleed before you started HRT).
There are other solutions eg a Mirena coil which delivers a steady dose of progestogen to the uterus without fluctuations but depends whether or not you are progesterone intolerant? Also there are COC pills which give oestrogen as estradiol - same as HRT, eventually suppress ovulation so levels remain more steady.
Even when you become post-menopausal, you don't necessarily need to make progesterone every day if it doesn't suit you, though you may find when your natural cycle has stopped, you are not so sensitivie as you will no longer be producing your own. The option remains to take it cyclically as several of us still do, unpalatable though that may be!
Hope this helps
Hurdity x