Rosanna, the thing about perimenopause is that our wretched hormones don't follow the predictable monthly pattern they should in earlier, more fertile years, so it can be impossible to know what is going on at a given point! That said, I'm sure Hurdity or another knowledgeable lady will be able to point you in the direction of a good technical guide, if there is not one elsewhere on this site (sorry, I'm a relative newcomer here).
As regards your palpitations, a few years before peri started I found myself having cardio investigations for these which at that time, seemed to occur in the 2nd half of my cycle just before a period was due. I tentatively asked the Cardiologist whether they could be due to gynae hormone activity, but he didn't think so. A decade later, past the time when these had really ramped up & become quite a worry during the chaos of peri, a well-respected Gynae told me she had previously worked in Cardiology, where many middle-aged women had presented with palpitations, had the full range of investigations, only for no abnormality to be found & peri/menopause to be the most likely cause. I wished I had known this at the time - it would have been so reassuring! Mine did go away within a couple of months of starting HRT post-meno, but a 2nd thyroid hormone I need had also been introduced about the same time, so we were not entirely sure which solved the palps. As mentioned before, a recent change of HRT brought the palpitations back with a vengeance & reverting to the Evorel Conti I was on before this, seems to have settled things again. So for me, gynae hormone fluctuations do seem to be at least part of the cause. If you are having long bleeds, anaemia can also be a cause, though I think you said your FBC was fine.
Some ladies recommend charting symptoms (I believe CLKD & Elizabeth Rose have found this helpful), as this way you can begin to get to know signs from your own body that may help you identify patterns & anticipate what will happen next. This kind of self-knowledge can be empowering, and may be enough to help you through without resorting to HRT, if this feels more comfortable to you (assuming symptoms don't become too disruptive or go on too long). I'd also been charting symptoms before peri began, at the request of an Endocrinologist who was helping me with the thyroid condition, and found this does help to a certain extent.
That said, I found peri a very difficult stage to live with, not knowing how long symptoms would go on, and with hindsight wish I had persevered following a failed 1-month trial of HRT in the early years, which my then GP incorrectly told me was long enough to know whether HRT was right for me. Because of this, Mum's breast cancer & my thyroid & migraine issues, I wrote off HRT as out of the question & as a result wasted 10+ years until, 3 years post-meno with good medical advice & support & having learned much from the ladies here, I decided to give HRT another try. I am so glad I did & would encourage any woman whose symptoms are severe to consider it, first learning as much as they can about benefits & risk. In my case, HRT hasn't got rid of all meno symptoms, but I am better with it than without & many ladies do feel far better with the right HRT. I am hopeful that there might be a better regime for me, but if not, I will probably continue with the Evorel, bearing in mind the thyroid condition may limit potential for the kind of greater improvement many ladies enjoy, and which you may too, should you feel you want to go down that route. Sorry this is so long, but hope there's something helpful in it.