Big hugs to you for a start!
Have you tried an antidepressant? You have every reason to be depressed, cancer treatment and then not being able to work is very demoralising, exhausting, scary and generally STRESSFUL.
Whether you're on antidepressants or not, you have to pester your GP for more help.
They are mostly overstretched beyond their ability to give the statutory services, so if you need to, get someone else to fight to get hold of a doctor for you, people are being left without life saving services, but not everyone is.
You need to be able to fight your way to the front and it sounds like you are too easily pushed away and not receiving the help you need and deserve and have paid for all your life.
It's true, each type of HRT is meant to be tried for three months, most women will have a good idea whether it's going to be successful by the end of two months. Your doctor ought to listen to you and stop wasting your time like this. It's your life that is being taken apart, or more to the point you haven't been able to pick up the pieces yet because your GP is being stubborn about a very arbitrary guideline which applies equally to vastly different types of HRT. I doubt that three months is based on anything scientific, more that it's easy to remember if it's always the same!
Hormones can make anyone's life hell, your doctor has the ability to give you something that will help you, don't lose hope because she hasn't managed to do her job successfully yet, most doctors love to hear they've helped, they just need a nudge to get going.