The description of the evolution of your problem through the teenage years is very similar to mine. I didn't realize that the issue was privacy until I was about 18, a little over 10 years ago. Before that, while I was often uncomfortable, I was blissfully unaware of the real problem.
I'm now focused less on becoming un-shy and more on just accommodating the problem. When I travel, I will often rent a cheap room just for myself, near to the room I might be sharing with family/friends. This allows me to disappear in the morning or as I need to relieve myself. Otherwise, I will not enjoy the trip. Of course it can be awkward explaining myself, but my close friends and family know about my issue (I told them years ago). The awkwardness of explaining myself is not as bad as feeling trapped and dreading trips weeks in advance! I know just what you mean about stressing over trips beforehand.
I hope to have a family someday, and I've already decided I will have a separate place I can go when I need to, whether it's a guest house in the backyard or even something more drastic. It helps me to be optimistic about the future.
I'm also open to the possibility of my condition improving, but in the mean time, I will make the accommodations I need to feel good.