The HGA is (From what I can understand) simply you, and your essence removed from the world and its vices and virtues. It's as close to what and who you are from an essential view. this likely doesn't answer any questions, but we can also see that the HGA is connected to the Heart, and therefore we can say that the HGA is you, and listening to the HGA is simply listening to those things which resonate most firmly with your heart, indicating those things which you desire doing more than anything else. Those things which you feel an ache in your heart about, and those elements of life which you connect most firmly with. This is what I think it is to listen to your HGA, to listen to that little piece of you which is screaming those things which are so deeply ingrained in you that you couldn't change them if you tried.
True will is not much more than that. Disregarding everything surrounding you, if noone existed but you, and no circumstances were standing in the way, it is doing exactly what you most would be called to do following those pangs of your heart. Not speaking in terms of the connective nature of the heart, mind you. Those can be deceptive and cruel, tricking you into thinking that that nature is YOUR nature, and not simply the result of your connection being disconnected. No, the True Will does not hurt, unless you are ignoring it. The True Will is always uplifting and fulfilling.
This is my experience, and none of this is saying that outside circumstances should not be considered in the APPLICATION of your True Will, but finding it shouldn't take any other considerations than looking inside of yourself and searching.
No one and nothing can tell you what your "True" will is other than you. The HGA is not removed from you, but is an intrinsic part of yourself. To ask where it came from is to ask where YOU came from, and that is the purpose of many a system of study including Kaballah. I think your "True Will" has the potential to continuously change, but if you're consistently at the place of searching for guidance from the place of Tiphareth, then you will find that the basics of your Will are similar, and the only change comes from application due to surroundings.