Jessica, lethargy and tiredness in the first trimester can be debilitating. With all of my pregnancy's I found that to be true. My dd3 was the worst. I was a shift worker and it almost ended me I am sure!
The good news is that by about 13 weeks you will start to feel perkier. You are doing all the right things for yourself and your baby. Eating well is the best thing you can do for the tiredness and ensuring you drink plenty. Even though it sounds horrific, a gentle walk can actually help (it did me!). Have you had your haemoglobin checked this pregnancy? If not than maybe ask for you gp to check this. However, what you are describing is usual and is the result of your very clever body working hard to grow your baby. Nuts to nibble on (esp almonds) can help to give you energy. Floradix is a very natural iron supplement that I take in the first trimester to help pick me up. I swear by the stuff, it is available from the healthfood store or the health section of some big supermarkets.
I hope this has helped and I know that you will soon start to be much less tired.
Lots of love and baby blessings,
Deb