I would deffinately talk to your doctor about this and get some kind of professional help. When I had my second child things seemed to overwhelm me even more than the first because I had so much more to take care of. My daughter was a year and a half old when I had my son so that was quite a bit of stress right there. When you feel like you're at a point where you can't handle anything else and you're getting that anxiety, you need to take a break. I know having a brand new baby you can't just take 30 min to yourself, but if you start to panic, walk outside, take some deep calming breaths and try to steady yourself again. Also having a support system helps a lot too. If you have a friend you can call that will just help you laugh some of the stress off for 5 min. Laughter is great medicine. You may not think so right now but while you're trying to get things sorted out with your doctor you have to try and do the best you can at home. I check my inbox a few times a day so if you need to vent or need anyone to talk to I'm here. I hope things get better for you. Wish I could help more. I'm just trying to give you tips on what I did when I felt the same way.
The best advice I can give you is to call your doctor IMMEDIATELY. Don't wait, pick up the phone NOW and call. I applaud you for realizing that you are having difficulty. Let your doctor help you manage the ppd and anxiety. Now, with that said, take a deep breath and look around you. Ask yourself what do I absolutely HAVE to do and what can wait. Examples...you HAVE to take care of the baby but the dusting can wait. You HAVE to feed everyone, but folding the laundry isn't an absolutel necessity, atleast it's clean right? Being a new mother, especially if you have other children can be VERY overwhelming, trust me I know, I have 4 and one on the way and I suffer from depression. My next suggestion is to get out and get some sun. It's amazing what sun can do for a depressed mood. See if you can get someone to watch the baby for even a 15 minutes so you can go do whatever you want, even if it's crawling in a corner and bawling your eyes out...it will make you feel better. I also agree with the above posters suggestions. But, first and foremost, you need to contact your doctor.