OK, your GP is a dope. We realise that. Was this perchance in one of the big bulk-billing "medical centres" which the government is promoting?
You do not need to be concerned about whether your degree of tightness is "worse" than anyone else. For all we know you might be able to stretch much more effectively than average. It remains that if you put the time and effort into it, you will be able to stretch to the degree you need, but it will take time and effort. In particular, if this is a problem that has been present since childhood, then it is simply a lack of whatever it took to stretch it gradually to a normal function over time. In the absence of some skin disease that would cause a normally mobile foreskin to tighten over time, the betamethasone is not strictly necessary, though it does help a little.
The illustration Jim gives is not particularly effective in stretching as is, but is what you do in order to get the first finger inside and fully through the tight part (admittedly, not easy to do unless you have three hands available - and that is a serious suggestion!).
As to the betamethasone, you will not be able to obtain it or equivalent without a prescription in Australia.
But - stretching will get you there!