Hello and welcome to the forum. Thanks so much for your question. The doctor you've seen is affiliated with the Army? How is this impacting your ability to serve? It does sound uncomfortable. I think a second opinion would be important to continue exploring what is happening. Your feet are newly flat? Different things can cause feet to go flat. Have you seen a podiatrist specifically? That's probably the best thing to do. There are different TYPES of flat feet as well. That will help you understand exactly what is going on and what to do if you can have a podiatrist determine the type of flat feet you have. An orthopedic surgeon may help also. Flat feet can cause misalignment elsewhere so it is no surprise you are having pain. If you are extremely active and physical, it's more likely to occur due to foot and ankle injuries. If you are obese, also more likely to occur. Do you use any foot support? https://www.healthline.com/health/pes-planus#treatment
It's possible a lot of different things can cause such things, but it might help to have a sports injury specialist look at how you stride when you run and how you lift when you're carrying equipment. Might be something off there. I can think of a lot of things that can cause hip, back and knee pain as I'm one of those old guys who is suffering from what I did when I was young and you're right, when you're old those surgeries don't even work all that well -- they work better when you're young. But when you say your arch has crumbled and your leg is turning in a different direction, that goes beyond the common injuries unless you started radically altering your gait when the pain started or you've got horrifically bad shoes. Your docs know a lot more than I do, but hip problems can be labrum tears or bursitis or both, or a periformis injury, almost everyone gets lower back problems at some point, and the knee could be torn connective tissue. You would seem to be too young for arthritis, but who knows? By the way, 45 is really young -- the bad stuff hits you later than that unless you're a serious athlete, in which case it hits you all the time. But again, if your whole body structure is changing you really need to have someone with expertise look at how you're doing what you're doing as it might be the cause if you keep doing it over a period of time.