BMR vs TDEE confusion, please help!

So, my best friend & I were talking today, trying to decipher the major differences that all of these calorie systems give you as what your intake should be. I've been told under 1200 calories you are basically going to go to 'starvation mode'. I was barely eating 800 calories before I started paying attention (not on purpose, just because one of my bad habits is forgetting to eat). I went to a nutritionist to try to figure out a plan, and she calculated that I should be taking in an 1800 calorie balanced diet. Because I'm also PCOS, I place major importance on how many & the types of carbs I'm taking in. But here's the thing, the nutritionist tells me 1800. My calculated BMR is (taking in account lean body mass) 1391.. I did the two calculations as I saw on one of the message boards and at my goal weight, leading a sedentary lifestyle (which I don't but this is closest to the calories my nutritionist advised) it says I'd need 1822 to maintain, my TDEE at today's weight (if sedentary) is 2092. I feel like these numbers are all over the place!!! Then I noted that you can reduce your TDEE by 20% if you want to lose 20+ lbs, which I do. And this, puts me under both of the numbers calculated, 1674, but slightly above what I've been eating the past week. So my quandary. What is the number out of all of these to focus on? I'm lost, confused, and concerned.