I so understand how you feel. I had Covid early 2020. I was debilitating sick for two years. Lost 30 pounds, down to 74 at the worst, 5'2'. Gained a little the last 6 months but still 89 lbs; used to be skinny at 108 lbs. All my muscles are atrophied and I am so weak. Naturally I was in the worst mental state. I was put on a low dose of Zyprexa to address the depression and to increase weight, that is how I gained ab. little. For the extreme anxiety I took a probiotic, Lactobacillus Rhamnosus. There are plenty of studies describing its effect on anxiety and the gut. It really helped the anxiety. I hope you can get it, I am so glad I found it.
All the best to you!

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I'm trying to break out of this vicious cycle myself: high covid anxiety, stress, lack of appetite, weight loss, stress over the weight loss and covid anxiety. I lost 50 lbs in 2 years and ended up with no appetite, and was losing 1-2 pounds a day when I hit a crisis and started reaching out for help. I went to a long covid clinic and the message I got is that stress makes everything worse. Stress can cause long covid symptoms to appear, and if you already have long covid symptoms, stress can make them worse. Lower the stress, lower the symptoms. Also, you can't treat long covid, you have to treat the symptoms, whatever they are, the way those symptoms would normally be treated. I'm seeing a shrink for the anxiety and a nutritionist for tips on proper nutrition. I have put on 10 pounds since my low - it became a project. I found if I can manage to put on one pound, I can do it again, and once you put on 5 it gets easier, and if you get to 10 you are feeling much better. You may still not have appetite, but it gets easier to eat. Please see a nutritionist – they can tailor a plan for you. In the meantime, here are some supplements that are easy to ingest and will help maintain weight. Pea protein is easily absorbed by the body - any brand would do but I used Arbonne and it stopped the slide for me: https://www.arbonne.com/us/en/arb/patrickansah/shop-all/nutrition/feelfit-pea-protein-shake-simply1---chocolate-flavor/p/2059%20US and greens https://www.arbonne.com/us/en/arb/patrickansah/shop-all/nutrition/wellness-support/bewell%e2%97%8a-superfood-greens/p/6232%20US

I've had a nutritionist suggest these also, most available online only:
ENU Meal Replacement Shakes + Nutritional Weight Gain Drinks https://enu-nutrition.com/
Lutrish is a powder to mix in smoothies/milkshakes https://lutrish.com/
Naked Nutrition: Chocolate Vegan Weight Gainer Supplement https://nakednutrition.com/products/chocolate-vegan-weight-gainer
Soylent (may be in stores) Meal Replacement Shakes https://soylent.com/collections/all-drinks
Vidafuel One 2oz ‘shot’ size of this drink provides 90 calories and 16 grams of protein. https://vidafuel.com/