Oh. At first you said you were 10, and you didn't sound as young as 10. I don't know that flat feet cause anything bad to happen in most people, other than a higher than average occurrence of plantar fasciitis. (But so do people with very high arches, so it goes). I have a similar thing, though I'm a lot older than you are, my right foot sticks out to the right. Don't know when it started, but I do have a pelvis that is permanently a bit tilted and I've had a lot of injuries that weren't dealt with very well. But here's what I'm confused about with you -- an MRI should show if you have a meniscus or MCl problem. Manipulating your leg, as it appears has been done, also tells a lot. Because you've quite young, I'm thinking it's not likely to be a wear and tear injury and so there should have been a point where you first noticed it. Was there? If there was, what were you doing at the time? You say you're an avid skateboarder, do you always push off with the bad leg? Usually when this kind of thing happens you get analyzed and then sent to physical therapy to see if it can be fixed without surgery, unless an MRI shows an obvious bad tear that has to be operated on. That doesn't appear to have happened in your case. The first thing physical therapists usually do is get you to stop doing whatever it is you're doing so they can work on strengthening the areas above and below the injured area to take the stress off of it. If the injury is chronic, you may have to find something else to do. though not usually at such a young age. I can't tell you at all what's going on, but I have been through the process over and over as I've been very active and got old and this stuff adds up over the years. If it is bad mechanics, it's good to get it fixed early in life, but only someone with expertise in sports medicine and movement can do that, not just anyone. As always, if the docs you've seen aren't giving you what you think is accurate info, get another doc.
Here is a sad fact, some activity is hard on us and we can't sustain doing it. Ask any runner that eventually had to give it up due to their knees. I have competitive runners in the family (sons) and I will say that shoes directly affect just about everything. Depending on the shoe issue, they will have pain in knees, in feet, in calves. No joke, shoes they've gone to a specific store to purchase shoes from that have been fit to their feet usually solves the issue so it taught me to never underestimate having the right shoes. There are also telling you that your hips are weak. Can you not strengthen those by working with a trainer and/or physical therapist? This is something a lot of athletes I know have issues with, IT band syndrome. https://www.medicinenet.com/iliotibial_band_syndrome/article.htm Anything like that look familiar to you?