My melasma has increased dramatically since I started running.

I used to only do breathing exercises, but I recently started running. Since I run outdoors a lot, I get exposed to UV rays frequently. On top of that, I'm prone to dermatitis, so I can't apply sunscreen properly and have to rely solely on a hat for coverage. As a result, I get melasma even when I only run when the sun isn't out. ㅠㅠ Are there any products that can help prevent melasma? I've heard that Vitamin C serum is good; if you know of any good products, please recommend them.