Wow. Going back to work at six weeks and having to leave your baby really does seem like the kind of thing that would make anyone very *naturally* sad, which you are probably experiencing on TOP of what looks like possible PPD.
Many women reconsider going back to work once the baby is born and they actually experience their bond with their baby and also when see the nature of the situation, practically, as well. Because I'm a SAHM, my husband and I are a one-car family, the floor needs fixing after a botched concrete staining and we haven't been able to afford to replace the woodwork, either. There are many things we can't just go out and buy, and we do without. And I couldn't possibly care less because I know my baby and we spend our days and nights together, happy, sad, tense, tired, playing, exploring, and experiencing this unbelievably short time together minute by precious minute. And try as I might, exact sounds of her vocalizations and the way she looks at me as she's nursing does fade quickly, but at least I have it in the moment when it is there.
See if you can get out of it. Talk to your husband about becoming a stay-at-home mom, if this is what you really want to do. Look at your expenses and see what you really need and what you can do without. You say you're badly in debt. Do you have cable? Trust me, you won't have time to use it much. Have it shut off. Can you start an at-home business after you have gotten into the swing of things? (You really do, you get the hang of being a parent pretty quickly.) Can you go part-time, either at your job or starting another?
If you want just a little longer, in fact, your doctor can write a note that your breastmilk supply is tentative and your baby is allergic to formula, so you need more time to stay home and nurse, and it would be the truth. Babies are all technically allergic to soy and milk. And early on, your breastmilk supply IS tentative! (I read this in the Dr. Sears Baby Book.) You might not get a paid extension, but at least you can get an extension.
If you've been using formula, consider working with a lactation consultant and La Leche League leaders (all free of charge) in your area to bring back your supply. It is absolutely not too late. This will save you and your husband roughly $2,500 in formula expenses. If you're using disposable diapers, sell what you have on Craigslist or Ebay and buy cloth diapers (they're very easy; check out Bumgenius 3.0, for example; you can find good deals at diaperpin.com, among other places, but avoid eBay). This will save you about another $2,000 or so - I forget the figure. We've only spent about $200 on ours, and our daughter is already 8 months old! She will use most of them till she is potty trained; others aren't one-size and she'll outgrow them but they last a long time, too. Then of course using cloth baby wipes instead of always having to go out and buy more saves you even more money, God knows how much. And of course, if you stay at home with that lovely little child of yours, you save money on childcare expenses.
Go to thrift shops for his clothes (and yours!) and don't worry about extra furniture. Have him sleep with you; it's safer than a crib, if you ask me - and a lot of other people out there, including the experts.
Don't buy jarred baby foods when the time comes. Make your own. You'll do wonderfully. Get Super Baby Food from the library and see how easy it is. You can read it as you nurse :)
Be creative and see what else you can do, if you decide you want to be there with your son. You might be amazed at what expenses crop up with going back to work, anyway, that make it not worth the pay: clothes, lunches, gas spent (projected to increase in price again), daycare, and of course days off of work when your son gets sick - and kids in daycare get sick constantly - medical expenses when your son gets sick, etc.
If you opt out of your current job for a part-time position with them or another company, you can look for a mother's cooperative group where women take turns caring for their children, and it's either cheap or free. This will reduce illnesses/med expenses, and is usually much higher-quality care than daycare.
I am willing to be that you have more possibilities open to you than you think!
L.