As kelly said the ability to gain and maintain an erection varies, but very commonly there is a loss of erectile function following prolonged HRT.
That said a good endocrinologist should be able to tailor your regime to help you.
I am very surprised that your Dr didn't warn you that loss of erectile function is a common side effect of HRT. Did your Dr warn you of other possible side effects?
DVT, feminisation, mood changes, etc?