I know it probably doesn't it feel this way, but you actually don't have that much weight to lose. 20 extra pounds is just kind of a comfy weight for your body…you're happy there, so you won't drop fat by being super restrictive…1000-1200 calories is too restrictive for you.
It doesn't seem like you're eating enough. Here's what I would do:
1) Make sure you're getting enough water in.
2) Keep an eye on your sodium (I would try and stay around 1500mg)
3) Increase your calories by about 10% each week until you are eating your BMR - and then stay there for about a month before manipulating the numbers any more.
4) Try not to backload your calories. If you want to eat three meals a day and hit 1200 calories total try and go 400 at breakfast, 400 at lunch, 400 at dinner, (as best as you can, of course).
Mini Goal #1: 169lbs
Mini Goal #2: 164lbs
Mini Goal #3: 159lbs
Mini Goal #4: 154lbs (LOWEST WEIGHT AS AN ADULT)
Mini Goal #5: 149lbs
Mini Goal #6: 144lbs
Mini Goal $7: 139lbs
Mini Goal #8 (Maybe): 134lbs