K.
As my mom keeps reminding me... you can have it all, but you can't have it all at ONCE. At various points in your life you just need to shift your priorities.
Some ideas to consider:
1. Do you need to go back to work right away or can you wait a year? Your career hasn't really started yet, so now is a really good time to stay at home for a year or two if you can/want. Much easier now than it will be to go back to work for a year, then drop out again.
2. Things will get much much easier as your baby gets older - there's a *big* difference between a 3 month old and a 6 month old. You will find more time to keep the house clean and exercise soon. Better yet, try and incorporate exercise into your daily activites (I ride my bike to work every day) and you won't need to spend extra time doing it.
3. What time does your baby go to bed at night? If you can get him to have a ~7 PM bedtime (my son has been doing this since he was 2 months old) then you'll have some "me" time after he goes down.
4. Pick activites to do with friends that are low key. Invite them over for dinner and a DVD. Play some games. Go out to lunch on the weekend. All of these can have a baby easily incorporated into them. Especially if you're at someone's house, it's easy to put the baby down for a nap/bed and have some time to be an adult.
5. Don't worry too much about the dieting thing just yet. Try and eat healthy. For me, this means that I go through a cookbook and plan 3-4 dinners to cook for the week. Then I grocery shop on the weekend to make sure I have all the ingredients. (We eat leftovers for the remaining meals). I eat better, and healthier, than we would otherwise. If you're breastfeeding, don't bother dieting at all - that weight will drop right off, just be patient.
It's hard, but don't try and be a "Supermom". There's no point in burning yourself out. Who cares if your house is a little messier than it was? And nobody expects you to look like a model when you've got a 3 month old.