You have some pretty good advice from the other moms, but I wanted to add a few tips too. I took maternity leave for 4 months, and every day was really rough, being isolated at home with a crying, hungry baby.
The things that helped me were my mommy & me yoga class - the moms all brought their babies too, and nursed mid-workout if we needed to. It gave me some comeraderie with other moms struggling with their own new baby challenges, losing the weight, etc. And it got us out of the house every 2 days for 1.5 hours. I know our city recreation department even has a similar new moms stroller class that will likely do the same thing. Also, some of the church-based day cares in my neighborhood have something called "Moms Morning Off". It's 10 weeks of daycare on Friday mornings for $175. Something like that might give you just enough relief to catch up on your fitness, your house, or just feeling good.
The other big thing for me was to NOT sweat the small stuff. I figured out what was a non-negotiable part of my routine for my sanity and prioritized that. For you, if it's your sheets, prioritize the sheets (but seriously, if your family bathes every day, they're really not getting that dirty!) For me, that meant, putting my son in the swing for 15 minutes for me to take a shower, dry my hair, and put on some clean clothes. Even if he cried, or was bored, those were MY 15 MINUTES. It is a small investment in your mental and physical health, which will be healthy long term for your baby too. The other thing that my husband & I HAD to do was designate one evening for me to have as my own. Some nights, that meant putting in ear plugs for a few hours to take a nap. Some nights, that meant me sneaking out for 2 hours for a drink and dinner with a friend while he fed the baby & even folded some laundry. Even if you're breast feeding, your baby should be able to go 2-3 hours without a feed. Dads can figure out how to cope with a few diaper changes, bottle feeds and putting down for a nap. They're parents too, just not mom.
Also, see if you can enlist help from your kids in some household chores. If you're like me, it's not a problem to get the laundry cycled through the washer & dryer, it's folding it all & getting it put away. Even my 3 year old can sort his pants, shirts, socks & undies and put them into his dresser. Do I care if they're folded? No. Don't sweat the small stuff! You also need to make sure your husband is helping. Paperwork or not, you should not be the only parent bearing the burden of a new baby. Identify the highest 1-2 priority things you want him to help with, and have him do that. Maybe it's brushing your daughter's hair every morning, or maybe it's taking your 3 year old to the park for some quality dad time one evening per week.
Figure out what battles you want to pick & enlist some help in getting your top priorities done. And if you still are feeling down, and like your situation is insurmountable, go see your doctor. PPD is a serious condition that can impact you and the rest of your family. Get it taken care of.