Has my metabolism been destroyed?

Okay, so just as a little bit of background info: when I was younger it was not unusual for me to go into temporary starvation for a few days at a time - probably no more than five days a fortnight, but those were sometimes all in a row. It wouldn't always be deliberate - sometimes there was no food in the house, often I would be late to get to school, have no lunch money and have stayed out too late to have dinner at home, but occasionally it was a choice as I suffered from depression and often struggled to eat. I was never sporty, but I was always fairly active as I used to go clubbing and partying a couple times a week and would walk a lot as I've never had a car and rarely had bus fare but always wanted to be out of the house, hopping from friend to friend. I probably walked four miles a day most days getting to and from places.
When I was 18 that flipped round. I was living with friends so instead of staying away from the house and in turn, the kitchen, I was able to chill out on the sofa with pizza whenever I liked. I made bad food choices and was living on campus with taxis that were cheap and busses that were cheaper and I ended up gaining 50lbs in two years.

And the actual question: I've now been trying to lose weight by working out a calorie deficit of 500 a day. Due to my height and level of activity that means that for the last month and a half (before ten days ago) I was losing steadily and only netting around 1,100 cals a day.
I'm 5'3 so this shouldn't have put me into starvation mode, however I've noticed that my brain wasn't performing well on that level and that I got cold a lot faster, which is not at all helpful during this freezing cold exam period, so I agree that despite the fact that I wasn't starving, it wasn't healthy.
I decided to up my calorie allowance and to 'maintain' over christmas and during exams, so upped my calorie allowance to 1,600 (the amount MFP thinks I burn throughout the day) over the last 10 days. But christmas got the better of me!
Once I eased up on the self restraint a little bit, it all fell flat.
Some days I've gone all the way up to 2,300, some days I've been only at 1,000, but the average amount has been 1,945 - so 345 over my recommended calories.
However, in ten days I've put on 6lbs. My understanding is that usually, 1lb = 3,500 calories, right? So I should have put on exactly one pound. Where have the other five come from??!

Have I ruined myself? And if the answer is yes, which I'm sure it is, how do I get out of this and back on my weight loss journey?