You need to speak to your doctor and they should arrange some help for you,do it now before it gets really bad..baby is only 3 weeks and not likely to be sleeping through the night for another few weeks yet..its not easy and hormones are still all over the place but hang in there hun it will get easier and there is help available so dont be afraid to ask for it and rope in any family members or friends to help out if you can even if its just so you can get a few hours sleep
I had my postpartum depression at 32 weeks with my twins, times were rough and all I did was cry, I finally went to my doctor and he put me on prozac. I'm not the type to take any prescription drugs but with the extreme way I was feeling I had no choice. Good luck mama it will get better, I promise!