HELP-weight loss slowing down at a crucial point

Hi all. I am have been trying to improve my eating and lifestyle habits since March and have lost 20 pounds attending Slimming World and following their plan (although not always to the letter). I am now at 160 pounds and would like to get to around 140. I'm 30, female and 5,6 btw.

My weight loss had been going really well over the summer with consistent loses of 2.5-3.5lbs per week. As I am off back to uni to do a masters at the beginning of October I upped my game over the last few weeks cutting calories and exercising a fair bit more in a effort to continue losing around 3lbs a week. However the opposite has happened and my weight loss has slowed right down. I lost 1.5lbs this week and 0.5lbs the week before. I have a number of theories about why this has happened:

1. I am pushing myself too hard and have gone into the starvation mode I keep reading about-my average net calorie intake after exercise was around 500 (only since joining MFP this week had I heard of eating back your exercise calories)
2. I am grossly overestimating my exercise burn-I don't use a heart rate monitor
3. I consume too many of my calories from sugar-I have a really sweet tooth and regularly have double the recommended sugar levels on MFP and this isn't just from fruit
4. My weight loss is inhibited by the fact that my net calorie intake varies a lot across the week-for example my lowest net intake this week was around -1000 and the highest was around 1700. I am a bit of a binger (I hate it but do struggle a bit with this and have throughout my weight loss journey-it is getting better but it is a slow process)
5. All the stressing and obsessiveness about weight loss has actually inhibited weight loss through the production of cortisol (a bit of a left field theory but it has crossed my mind)

If any one had any thoughts on my situation I would be extremely grateful-I am a bit confused and frustrated at the moment and am not sure whether there is anything I can do to get back to previous weight loss levels or whether I just need to accept that 0.5lbs-1.5lbs are good loses and be patient.