Stuck on a 2 Year+ Plateau, please help!

Female, 20, 5'7"
Starting weight: 200 lbs.
Weight this morning: 150.2 lbs.

This is my first post on here so hello! Back in late 2011 I had finally had enough of being overweight and started to do the Atkins diet. I went from around 200 to about 155 by the very end of 2011. I understood that no matter what diet you have, you can't just eat/exercise that way and then stop and expect yourself to stay the same weight. That being said, since then I've always been very conscious of what I eat. I won't lie, there have been countless times where I know I've eaten too much or that I wasn't eating the best of foods. If I was given the option of sugar-free or just a healthier choice I was all over that though. My weight since then however has never exceeded over 158.
The past couple of months I've been "dieting" again, I'm not on Atkins again seeing as how its a bit more difficult for me to do at the moment but I try to eat a salad as my dinner (with vinaigrette as my dressing), I always have oatmeal in the morning with a little bit of peanut butter (mmmmmm) and for lunch I either have brown rice, veggies and some meat or I make sure to have the recommended serving of something like mac n cheese and a corn dog. I've also been jumping rope for about 30 minutes 6x a week and I go on walks sometimes. At the beginning of these couples of months I was losing about a pound a week and now I'm down to 150. However, for about the past 3 weeks my weight has stayed the same. When I log in my food and exercise on here I always end up with about 300+ calories to spare (This site suggests I eat between 1,300-1,400 calories a day).
I'm just so frustrated that in 2+ years I essentially haven't lost much weight. If anyone has any ideas as to why this is happening I'd love to hear them, or if anyone has any advice please let me know!