I really would appreciate a supplement or something that can help my metabolism slow down to prevent weight loss even after consuming 2, 800 calories.

Please stay in touch. w/your doc. That's a low amount of calories. Some folks find a ketogenic diet helpful. It's fat burning but ask your doc first. "Keto Clarity" by Jimmy Moore is a good book in that regard. Peace and good health. Good fat is good for us. Beware of low fat misinformation.