What does your diet look like?
I'm sure you're already aware, but diet is a very important part of building muscle. Don't just 'eat more', look at what you are eating. You should aim to eat quality foods. Try and get five portions of fruit or veg per day, it's not that hard once you find things you like. A tin of mackerel will be much better for building muscle and health than two snickers or a sugar filled protein bar.
If your diet isn't great, that could be why you're feeling tired during your workouts. You'll struggle to find a better breakfast than porridge. Before you work out make sure that you have eaten some decent carbohydrates, whether it be porridge, pasta or potatoes.
On days where I eat processed crap with little nutritional value it absolutely hits my system. I feel lethargic and not motivated to exercise.
In saying that, don't over-complicate it otherwise it's unlikely to last. Find a few healthy meals you like and can cook. I literally chop up vegetables I like, throw them in the oven for an hour with some olive oil and curry style seasoning. It's tasty, has at least five of my five a day but most of all, it's easy.
Also, as others have mentioned make sure your getting adequate sleep each night.