The reason you are losing interest in the activities you are doing is because your diet is not a highly energetic one. Your lack in energy is only providing your vital organs with the accumulated energy you have to survive there is not enough energy for your body to live. So this is why you have no drive to do anything for the fact that your body doesn’t see any motivation to use any of your reserved energy that is for your vital organ unless it is to survive.

It could also be a sympton of Aspergus Syndrome, people with aspergus seem to go through different hobbies and interests every few months they can be very obsessive compulsive and be very obsessed with one thing for a certain time and then never do it again. but i generally dont think it is a sympton of ADD and it can also be due to your diet as stated above