Postmenopause just means that you are no longer ovulating as your ovaries have run out of eggs, and consequently your estrogen levels will be very low, hence the hot flushes and mood symptoms.
It is usually a clinical diagnosis when someone has been 12 months without menstruation (and no other reason such as birth control) but can also be confirmed by blood FSH levels above that particular laboratory's threshold.
Some symptoms of menopause improve over time - untreated vasomotor symptoms are often cited as lasting around 7 years on average, however others such as genitourinary atrophy and loss of bone mineral density are progressive.
If the hormone therapy you have tried led to nausea, you could try a non oral route eg patches or gel if you haven't already.