Wish I could help, I have been working full time for most of the summer (except for my holiday) and I am absolutely shattered, all the time. I find it that when I get home, there's not much I can do but lie on the couch for a while. I've been trying to get to the gym, but I only do it on my days off as I am too tired all the time. Sometimes I can manage to go out (about once a week) but I am really exhausted the next day and if I have to work, I'll just do a terrible job.
Honestly, I am wondering how do people do it? It just feels like a full time job slowly takes over your whole life! And there's nothing you can do...
In a couple of months you'll get used to it, but the thought of going on like this for the rest of my life really kills me.
When the term starts again, I won't have to do it anymore, but it does terrify me that that's what's in for me after I finish my studies.
Not sure the diet thing will help a lot - after all, I have a relatively balanced diet, I don't eat any white bread at all, lots of veggies, some fruits, yoghurt, milk, eggs etc. but I am still tired all the time. But I agree with staying away from caffeine.