Does anyone know if there can be a medical reason to keep you from losing weight besides thyroid problems?

I rode last summer consistantly and then fell off for a bit because of weather and then DH bought me a treadmill and I have been on it with maybe a day or two in between. I watch what I eat. I kept a log for about a month and went to a dietician and she said I wasn't eating enough and had me on this exchange diet.
I still have not lost weight. I weigh 230 lbs, shoot if anything I have gained more weight since last summer.
I quit smoking last summer in fact today is a year being smoke free, and I did this to become more healthy and I have slowly gained more and more weight even with all the exercise and watching what I eat.
It seems like the harder I try to lose weight the more I seem to gain.
I'm tired of going to the doctor and getting no results and feeling like they just don't believe me.
In fact most people in general do not believe me when I tell them I am exercising and dieting and I am personally really tired of it.
I'm miserable and feel fat (which I am) and I have problems with my feet and ankles (planters fastitis sp?) and I am quite sure the weight does not help as far as this goes and also the arthritis that I have.
I would like to get healthy and be a healthy weight sometime before I die, I mean it sure would be nice to wear clothes without buying things just to cover up.
I just can't figure it, and I get so frustrated.

Of course I am not going to lie, I cheat on ocasion, but nothing to extreme and I really try not to call it cheating so much as just allowing myself a treat once in a while.
I'm to the point of going to some quack doctor somewhere that will give me drugs to keep me from eating. I mean it feels like I am just going to have to starve in order to lose weight.
I'm so sad sometimes. I love to ride my bike and walk on my treadmill. I feel better after I do it but sometimes I really have to force myself to do either because I get into my pity party and start feeling like "what is the point".
Okay, sorry, I didn't mean for this to become a book.
I am just at my wits end.
I guess I will just eventually have to reside myself to being a healthy big person.