Homeostasis, and natural weight?

Ive seen a few things on the internet about our' natural weight' as predetermined by genetics. Im really starting to buy into it based on my own experiences. i believe my natural weight is 180lbs.

The highest weight Ive ever been was 240lbs about 18 months ago, which I got to through exam stress as was in my final year at university. mega stress eating!! I lost 70lbs through eating a bit less moving a bit more- never counted a single calorie and got down to 170lbs about 8 months ago.
It wasn't that hard to lose the weight if Im honest - I definatley made a conscious effort to make better food choices and to do more exercise, but I still ate badly on occasion and generally felt i lost the weight naturally i didnt deprive myself at all.

For the last 8 months Ive been in and around 180lbs - very little has changed in my diet in fact Ive been going to the gym alot more my fitness is better than ever, and my clothes size is smaller at 180lbs than it was at 170lbs. So im healthier than I was but the weight aint going nowhere!

Then in December I binged out for 1 whole month just didnt care anymore had some stress at work, lots of christmas parties loads of alcohol and was eating like 4,000 kcals per day on average, for about 6 weeks I ate like that and I gained 10 lbs when i weighed myself the 2nd week in January. I then lost 8lbs in one week after I started eating a more normal amount so back at 180lbs agian like that- easy after 6 weeks of damage 1 week healty eating and im back??!

For the last 5 weeks been religioulsy logging calories, MFP set for me to lose 2lbs a week so my target is 1230
About 50% time im below my 1200 normally on exercise days, which is normally 3-4 days per week.

50% i'm above my 1230 - however only on 2 occassions in the last 5 weeks have i ever exceeded 1800, so im not over eating, or undereating, im doing weights and cardio, at the gym - and im varying workouts w/ fatburning workouts 1-2 x week, HIIT 2x weekly weights some resistance training and some classes bodycomabt/zumba etc. I'm eating a balanced diet - but not losing any weight at all.

Im 5'9 female and my ideal weight is probably around the 150lbs mark, but I honestly cant see myself ever getting there, i think 180lbs is my natural weight when i overeat or undereat oveer exercise or under exercise i always end up about there I think its to do with homeostasis and balance, when Im eating alot my body is using the excess fuel efficiently, when im not eating as much my body is holding onto the every calorie and storing energy.

Has anyone else had similar experiences? Do you believe in a genetically pre-determined weight or do you think that if you accurately calorie count for long enough its a physiological certainty that the weight will drop?