Let's assume that you have been given a clean bill of health from your doctor (e.g. metabolic problems such as diabetes, thyroid problems etc. have been ruled out), except that you are overweight. The medical advice given to you is that you should lose weight via dieting and exercise. The problem now is that if you attempt to do that, you feel tired, have a lack of energy that is interfering with your ability to concentrate at work.
The best thing you can do is to visit your doctor again, but this time stick to this advice, which in this case means that you should write up what you are eating, how much exercise you get, what your sleep habits are, and what your energy levels are day by day. Make clear that things are not going well and that this is a persistent problem as demonstrated by your records. The doctor can then assess whether new tests are necessary, because a clean bill of health may only refer to having ruled out the most common metabolic disorders, there may well be other not so common medical problems that have not yet been ruled out. The more detailed information you can provide your doctor, the better that assessment can be made.
If the doctor still thinks there is nothing wrong, he/she could refer you to a dietitian.