I don't think the cause of CFS is known, so it may not possible for you find out the reason for your tiredness, but you can figure out how to deal with it in the best way that works for you. It also could be possible that you might have some other condition, so you might want to get checked out to make sure there isn't something else causing your fatigue. You're already working part time, so you've figured out how much you're realistically able to do, and you already know that you need to rest. Those things are working for you. It's unfortunate you can't work more and make more money, but maybe you need to try to improve your feelings of guilt, since they're only making you feel worse, and focus on finding things that are fulfilling to you without tiring you out, maybe watching movies when you're too tired to read, or reading something less deamanding, or doing small sit-down projects, like crafts or whatever you enjoy.