Congratulations!
As other moms have responded, breast feeding is a great way to loose the weight. It's naturally designed to be that way. My pre-pregnancy weight was 127. I gained about 20 lbs. while pregnant. 13 months after giving birth I weigh 117. And I'm still nursing, so more weight is going to come off. I'm pretty much hungry all the time. I eat 3 meals a day and lots of snacks. So, I am one of those who has a hard time keeping the weight on. The only exercise I get is walking with the baby, which I did several times a week.
Some people don't loose weight even though they breastfeed. I don't know why. Maybe they are giving formula, as well. Maybe they eat bad diets. Maybe they don't exercise.
You need to be sure to eat and drink enough of nutritious, healthy food, though, because you need the fuel and water to create the milk for your baby.
Don't ever take diet pills -- they are bad for you and especially bad for your baby (whatever you take in passes through your milk to your baby). You say you are an obsessive dieter, but still overweight when not pregnant. This makes me wonder about, in general. Are you the kind of person who chooses to take the stairs over the elevator? Do you drive around and around trying to find a parking spot up close, or do you take a spot that might be a little farther from the door. Little choices reflect our attitudes about being active and add up. Do you eat a lot of refined, processed foods? For instance, do you eat white rice? If so, switch to brown rice. Do you eat a lot of quick foods or do you prepare meals more from scratch? Some people like to limit fats, meats, dairy, carbs, and other things from their diets and have various results. I think you can't eat too many fruits or vegetables. Some people have had great luck with raw food diets.
After giving birth, women's thyroid glands can get out of whack, so if you are concerned about not enough or too much weight gain after birth, talk to your doctor. Also, some researchers say that postpartum depression can be link to off balance thyroid levels.
Winters here are long, cold, and discouraging. I think the worst thing is staying inside and letting it get to you. Bundle yourself and the baby up and go for walks outside when you can. Also, you can walk at the mall. The main doors open long before the stores do and lots of people are in there walking. You can window shop while you do it. Maybe you can meet up with some other moms and walk together.
Borrow exercise videos/DVDs from the public library; as long as you return them on time, they are free. Look for them at yard sales. Borrow them from friends. You can do sit ups, push ups, jumping jacks, jumping rope, squats, stretches, aerobics, yoga, pilates, and other exercises at home. Even if you do 10 minutes several times a day (e.g., push-ups in morning, some stretching a lunch, sit-ups with the baby sleeps in the afternoon, etc.), it adds up.
In general, be sure to get out of the house -- museums, libraries, malls, parks, La Leche League meetings, grocery store -- otherwise it can be a bit isolating having a new baby, especially when you are stuck inside so much during the winter. You know that already, though.
Good luck!