I'm certainly no doctor, but I'll offer what info I can from my own situation.
I gained 35 pounds shortly after I turned 40. I thought it was initially from a "meltdown" I had over not having dealt with some past issues. I dealt with them and moved forward.
I'm now 47 and during the past 7 years, nothing I did allowed me to get rid of those 35 pounds. I've 5'7" and at that weight was considered overweight to obese as well. I knew I had to get rid of the extra pounds because I'm high risk for breast cancer. I spent 2.5 years with a personal trainer 3 days a week, aerobics another 3 days a week...increased my strength and stamina...did NOTHING for the weight.
I, like you, have never been a big eater. In fact, my personal trainer made me start eating whole foods rather than my mostly liquid on the run diet I'd been accustomed to.
I hate dieting (in fact, I've refused to do so because I know I won't stick with it) so have always been in search of a 'way to eat' that will work for me. I've used almost every "diet pill" on the market from green tea to fat burners...stuff from $15 a bottle to $150 a bottle...even used Amberen for a month thinking it must be hormones. Still nothing....
So, here I sit on Round 2 of hhcg, Day 14 I believe...7 pounds lighter. I lost a total of 12 pounds on Round 1 (45 days), followed the stabilizing protocol to a tee...and had no issues on the maintenance regardless of where or what I ate (in fact, stuffing my face for the load phase for Round 2...I still didn't go over my 2 pound gain from my set weight).
Now, I don't tell you about my hCG success to make you feel bad. I know it doesn't necessarily work for everyone. But I also know there's a ton of factors to look at before you give up and resign yourself to an overweight life.
The type of hcg makes a difference.
Writing down exactly what you're eating (and weighing it!) makes a difference.
hCG aside, I'm guessing from your post that you'd start losing if you actually increased your calories. After my trainer (who had me at 1200 calories the whole time), I spent months reading forums and blogs (a LOT of great info over at Beachbody.com...yes, I LOVE their workouts and I hate working out!). But what I kept seeing here and there is the folks like you and I who couldn't lose no matter what they did. So they INCREASED their calories (healthy, lower fat foods obviously), and finally started losing weight. So, when I stopped with my trainer, I moved my calories to 1600...and started losing! So, just a thought.
Something you may want to try out. You may also check with a holistic doctor to see if your body doesn't react well to some of the things you eat. My girlfriend kept gaining and getting sicker and sicker. Doctors put her on a ton of meds. She ate healthy...grew up that way...tomatoes, veggies, raw foods and such...
Went to a Naturopath who told her the foods that she was eating (which most of us call the "healthy" ones) were killing her. That was a year and a half ago and she was at nearly 250 pounds. She's now back down to 170 I believe, off all her meds, healthy again and has her energy back, and has eliminated several foods from her life (wheat and such included so she's fairly gluten free). I'd have to call her to get more specifics.
Anyway, I'm sorry this is so long. I just hate to see people give up on their health and an active lifestyle if they don't have to. If anything in this message helps you at all to keep fighting, then it was worth the time it's taken to write!!