It is likely a combination of diet and psychological reasons. Getting a book about goal setting and learning to do so may help to give you more motivation throughout the day.
You can test the effects of diet on your energy levels yourself, simply by writing down how you feel after each meal and a few hours later. You may find that meals with a certain fat/carb/protein ratio will give you more energy while others will make you feel tired a few hours later. Personally, and I have heard several stories of others, that they were in similar positions to you and got *great* results from cutting down carbs (although this apparently doesn't work for everyone). If you have hypothyroid, then you may be given drugs to help if you go to a doctor, but I think if you can find a solution through changing your diet you will find it more permenant and satisfying. I really recommend you try the food test. Good luck
