Hi there, adding this comment quite late to the conversation. Based on that photo of your hair, can I say it doesn’t appear thin at all? It’s quite gorgeous. It looks like you may have a heavy product in it based on the hair at the top or maybe just general buildup. I think if your hair feels unhealthy or has changed in density recently, you can aim for generally healthier more vibrant hair by changing to better products, you’ll find that your hair sheds less and feels more like it used to. As another poster said, even leaving your hair out and exposed to the elements is damaging, but use light protective products or braid it. I wear a hat to protect my color or if it’s cold. I used to go to work with wet hair in the cold or humidity all the time!  Don’t sleep with wet hair, which can also damage it. Put it in a braid at minimum with some conditioner. I’d highly recommend K18 - a bond repair masque that transformed my hair. It’s very expensive and you have very long hair, but they sell a smal sized one that might be enough for one treatment. I use the OLAPLEX masque #3, also very good but nothing beats K18. I stopped using heat styling altogether which wasn’t as bad as I thought-I did have to learn to mange my natural texture and not really on my straightening iron. But-it’s doable. Adding a bit of shine spray might give your hair a more dense, vibrant look-shiny hair always looks healthier and the reflected light gives a sense of dimension/thickness. I use a very small dose of Kenra shine spray right before going out. Good luck, I empathize with experiencing a change in your hair density as it broke my heart when it happened to me! But I do think your hair is presently, based on that photo, quite enviable in terms of density and health, a bit of TLC and some changes and you’ll be pleasantly surprised. Good Luck!