I think you've probably started lower than you need to, but it's not a matter of having screwed up. The key here is trial and error: increase your calories to 1600-1800 for a couple of weeks (and take the extra calories in more veggies and lean proteins, not in milkshakes

) and see how you go. If you keep on losing 1-2lb a week, you're fine. If not, you'll need to readjust your calories. You need to figure out what works for YOUR body.
Btw, if you stall and don't want to further decrease your calories, you can always add in exercise, or increase the intensity of the exercise you're already doing.
And just as a point of reference, earlier this year I lost from 155 to 138 on 1800-2000 a day, plus lots of exercise. But it depends on lots of different things, so you need to experiment to see what works for you.