Well I'm a vet and not a human doc.. although I would like to have a good vet as my doctor...we look at the whole animal and try to give them just one diagnosis. Human docs are trained to look at people in packages: gastrointestinal tract, cardio, pulmonary, urogenital etc. I think this is half the reason it takes women so long to get to the 'perimenopause' diagnosis...we have to work our way through individual specialities.
Briony I'm 42 so 'young' as well. Although I think it started in my late thirties the crap really hit the fan when I hit forty. About 6-8 months prior to the onset of the bad stuff I lost about 45lbs rather quickly, and purposefully by giving up grains and sugar. BUT little did I realize at the time, I removed a boat ton of estrogen from my system by doing this ( estrogen was coming from fat tissue- estrone). I think I would have ended up in the same place, but think I drop kicked myself into this by the weight loss. Bummer.
Your doc and GP are right in one sense ...you're 'topping up' your absolute low levels of estrogen..but you're also reducing your own bodies production of estrogen by taking estrogen. It's a weird and complex system.
So some of us have symptoms of roller coaster hormones: headaches, tinnitus, PVC's, etc. These symptoms are often due to hormone fluctuations ( be they high OR low). We also have an absolute deficit of estrogen : think vaginal dryness, low libido, depression, hair loss, collagen breakdown, dry eyes etc.
Most of us have symptoms of both roller coaster hormones, PLUS absolute low levels.
By taking estrogen you are 'topping up' your absolute low levels of estrogen ( meaning your lows should never get too low I think they say most women feel 'optimal' when their lowest estrogen is above 200 or so...different studies, different findings). But because of the negative feedback loop..you are also lowering your bodies own production of estrogen ( if you supply it from outside your body, your body will stop producing as much). So taking estrogen should theoretically both 'flatten' the highs and lows, as well as increasing the overall basal level of estrogen.
Depending on the route you take your estrogen...the daily fluctuations in and of themselves can cause symtpoms. So say you take an oral estrogen...this causes a quick spike in your blood levels of estrogen, followed by a steady decline in your estrogen over the 24 hours until your next pill. In some women this is enough to cause them to get headaches ( or other distressing symptoms) because of the rapid shifts from high to low within the 24 hours. Thats why patches work better for a lot of women...more steady state delivery.
As far as progesterone goes, I think it's very much an individual reaction to all chemicals. Governed by our health, our hormone sensitivity, our body fat composition, our mental health ( although I don't think this plays nearly the role that most docs would have you believe), and our genetics. Some women feel great during their pregnancy when their progesterone levels are through the roof high. Some women feel like garbage all pregnancy long. And yet other women feel like the bottoms fallen out right after delivery when your hormone levels drop through the floor. Some women get baby blues, some women get post partum psychosis. Individual.