HELP !! Trying to figure out how much to eat to lose weight?!

HELP !! Trying to figure out how much to eat to lose weight?

I am trying to lose 20 lbs, i been at it for 3wks with nothing lost. I work out daily burning 500 calories on the treadmill on incline. I eat the right foods no junk drink nothing but water 24/7. Im 5'6 190 a size 14 jeans and 32yrs old. I have been keeping up with my calorie intake and cant get past 900 calories a day, i tried for 1500 the other day and almost died, i just dont have an appetite, plus im stressed from trying to lose wt and dealing with an underactive thyroid which im taking 100mcg of levothy, i been on meds for just a month now. My mom tells me she sees a diff but i dont, i think im losing inches but i want to lose weight. My question can someone who knows how to caluate how much i need to eat help me out?? I want to lose at least 2pds a week. I know i can burn 500 a day from working out thats 3500 for the week and take 500 from what i eat. So how much do i need to eat to prevent this starvation mode my body much be in???? Plus I just got some protein powder to help.

You may be blocking your own progress by obsessing over it too much. I don't know any "normal" weighted people that eat only 900 calories a day. Don't take this the wrong way, but happiness is a much better diet than calculations, powders and score keeping. Stressing will hinder your progress and that's a medical fact. Try eating smaller meals throughout the day if your appetite is not there for larger ones. I want to ask how is your appetite for life? They usually are related. I don't think you are eating enough to lose weight - it takes energy to burn fat and lose weight, and you are not getting enough. Please lighten up on yourself and you may find your body lightens up as well!!