For the entire last half of last year I ate really really well. Mostly lean meats, vegetables, and fruits with hardly any sugar or crappy carbs.
I fell off the wagon during my vacation to disney and have been completely derailed for the last 4 weeks essentially. I went from having almost unlimited energy and only really needing 6 hours of sleep a night to feeling like garbage and being tired all of the time. I attribute this almost entirely to the sharp rise in my consumption of crappy fast food cheeseburgers.
My diet has been sporadic, my meals random in time and composition including far too much fast food. My calorie intake has also been sporadic. I'll eat almost nothing one day, then end up snarfing down 4500 calories the next day. My weightlifting has suffered as well. I'm frequently too exhausted and crappy feeling to lift and I then feel stressed because I've missed an opportunity to improve.
Such a simple thing, how much you eat, what you eat, something we take for granted all the time, but it can change your entire training regimen. When you aren't well fed, and well rested, then you can't train as well, you can't pay attention as well, and you don't absorb knowledge as well.
This is especially important if you have limited training time. If you can only train twice a week then it's incredibly important for you to get the most out of each minute that you are training and part of that is going to come down to your nutrition. Remember, lean meats, vegetables, and fruits. A minimum of sugar and bread, and plenty of post workout protein! Eat well, train well!