Unfortunately the best advice I can give is quite boring. Every time I say this I get my head bitten off. The conversation goes something like this:
Me: Why don't you keep a food and activity diary for a week or so...?
Client: Why should I. I know what I eat. Do you think I don't? Anyway I tried that years ago and it didn't work.
Me: No, no. What I meant was lets have a look and see just where you aren't eating enough and where you are eating too much. You might not be eating a lot but you may be depending on one sort of food, like carbs, for your fuel. That might be why you keep the weight. It also helps me design a good exercise programmme for you. I can do best if I know when you are most active and most fuelled.
Client: mumble grumble....
The results are startling and very predictable. All clients who refuse to do the diary struggle to balance their food and exercise. Those who do so honestly find it is really suprising. Especially when they themsleves can start to see patterns in their eating they hadn't seen, or didn't want to. They also get a kick out of being able to second guess me and tell me what I was going to tell them.
Now I am not a qualified nutritionist but I do have enough quals in health and exercise sciences to be able to interpret eating habits and help people make sensible choices. I'd be happy to help you. I don't write diets. I give sensible eating advice and I do write exercise programmes for unfit and overweight people. So feel free to pick my brains. I'll help as much as I can.
Try my e-mail if you don't want to go public