It's not just the weight loss that'll make you feel better but by eating the correct foods.....generally foods with a low glycemic index, this usually makes the difference.
At times such as xmas, i tend to eat all manor of rubbish....all fatty, sugary foods which are high in bad carbohydrates. This would include bacon sarnies in crusty white bread, loads of pasta, biscuits and crisps....all bad for you !!! Add some serious alcohol intake into the equasion and you have a recipe for disaster.
This makes me sluggish and tired but as soon as i go back to work, i hit the salads, wholemeal bread, bottled water and porridge for breakfast and within a few days, i am back to normal.
1/2 hours excercise such as a very brisk walk at lunch time will also wake you up a bit.
Hope this helps and good luck with the weight loss