Hi Anna,
I'm sorry to hear about your new scars, and that you're in such a stressful position. I hope it's not too late for my reply to be helpful. While I don't believe anyone should feel like they HAVE to cover their scars, I know it's not always a safe option to open up about your self-harm to everyone involved, either. So here are a couple of ideas.
I'm afraid I'm not much of an athlete, so take what I say here with a grain of salt—but perhaps there's a makeup option that would work for you? Most cheap options won't stand up to the heat/sweat/etc., but if you can afford to, some professional makeup (especially high-quality theatrical makeup) might be able to stand up to the wear and tear of sports. Some might even work in the water, but you'd have to do your research to find options made specifically for that. (This will work best if your scars are relatively flat/thin... thicker, puffier scars or more extensive scarring might be too much to hide, even with good makeup.)
I can't tell from your comment what sport you're practicing, so I'm not sure if this is an option or not, but is it possible to wear slightly longer sleeves/shorts that might cover your scars without being too obvious (or making you too hot to practice safely)? Or maybe you could wear sweatbands, at least to cover the ones on your upper arms?
Depending on how your scars look, you might also be able to come up with some excuses just in case someone does notice your scars. (I imagine the arm ones would be easier to write off as being the fault of a pet or some sort of an accident—upper thigh ones are a little harder to explain away.)
I'd also urge you to reach out for some extra support, not just for coping with the self-harm itself but also for more extensive guidance for situations like this—because this may not be the first or last time you face something like this. Here's our resources page, which includes some websites and hotlines you might want to check out:
https://www.healthyplace.com/other-info/resources/mental-health-hotline-numbers-and-refer…
I hope that helps, and I hope things go well with your practice sessions and that you're able to maintain your privacy as well as your safety. If you have more questions/concerns, reply here or elsewhere on the blog. I'll be around.
Sincerely,
Kim