Hi Lily
I have noticed this a bit as well, I am 45. The reason that low estrogen can add to carrying weight around our middle is that fat cells store estrogen, so, it's a protective thing that the body is doing - increasing estrogen through these fat cells

Our metabolism also slows down in out 40's and any stress creates cortisol that also lays down fat and sugar for quick release during the fight or flight response. All things combined are not helpful to us. We can do things to help the metabolism and the stress related fat.
Targetting stress and eating regular meals and snacks throughout the day can help even out metabolism and keep blood glucose levels even, which should all help some aspects. As for the low estrogen part, talk to your GP again if you feel you would like to discuss HRT again. If it's really a no no, there are things you can do to protect your bones, Vitamin D and weight bearing excercise and the heart risk can be modified by making sure other risk factors are kept to a minimum.
Both the British Heart Foundation and Osteoporosis foundation will have info on risk factors for both so that at least you can be aware of them and how you can modify them. x