Heya. Are you on LJ? Your tribulations sound familiar from someone whose journal I found on the trigeeks group.
For endurance training, Chris Carmichael says you need about 50-60% of calories as carbo, 10-15% as protein, and about 30% as fat. To snack without throwing off a diet, get thee protein, fat, and a little carbo for a boost, staying power, and satiety (nuts, seeds, yogurt, cheese). But otherwise, base your diet on the principles of healthful eating: get lots of veggies of many colors, whole-grain and low-gi carbos, high-quality protein, not too much animal fat and plenty of vegetable fat, don't forget your essential fatty acids, try to avoid hydrogenated fat and high fructose corn syrup at all costs. I love Andrew Weil's no-nonsense explanations of macronutrients, micronutrients, and what your body needs. (drweil.com) Your low-gi carbos (see glycemicindex.com) will provide the muscle glycogen and blood sugar you need for your training so try not to skimp on them.
Unfortunately you kind of have to focus on one goal at a time. Build power -- and eat to fuel your body; OR lose weight -- and ease up on the training. There are some good articles on this site: http://www.roadcycling.com/training/ in particular this one: http://www.roadcycling.com/news/article900.shtml is about that plateau that athletes invariably hit for those last 5 to 10 lbs.!
Finally, remember that as a woman, your body needs fat for a lot of reasons including proper generation of estrogen, and it's going to hang onto that fat if at all possible. Below 20% bodyfat and you're infringing on your reproductive system; below 15% and you run the risk of losing your period and possibly even doing long-term damage. I have to recommend getting comfortable with your body image with some padding, unless your sport is important enough for you to take those risks.
Good luck!