What you're describing is so, so normal. It's that 'oh fuck, what have I done' feeling.
What no one tells you is that motherhood is brutal. I felt like I was being broken in two, my life thrown into absolute chaos. But the good news is that eventually motherhood builds you up again. Stronger. But it takes time.
I think it's important to let yourself cry, acknowledge your feelings. You've got a lot to celebrate (your LO, your new identity as mother) but you've also got stuff to mourn (your old carefree identity, your childless relationship with your DP). Modern society doesn't acknowledge the things we leave behind when we become mothers. Older cultures were much more understanding of the idea that motherhood is a time of transition - and we need some help in making that transition. Naomi Wolf's book Misconceptions is really good on this. It helped me make sense of what I was feeling.
Talk to your DH about your feelings. He needs to be there for you right now regardless of his own issues - and he will be. Don't be scared of your feelings, they are so, so normal. They don't make you a bad mum - in fact the total opposite, because you are acknowledging the huge weight of responsibility and the enormous life changes that motherhood entails. In feeling this way, you're showing how seriously you take you new obligations.
You will work out this new life, you will embrace this new identity and there will be so much happiness and fulfilment in seeing your LO grow and finding your feet as a family rather than a couple. Trust me, it goes from chaotic/terrifying to wonderful pretty quickly.
A few things that will help you on your way. Get out of the house everyday. Build it up slowly. Take your LO out in the pram & just walk up and down the street for starters. Then build up to longer walks, trip to a coffee shop, etc, etc. Seek out groups/coffee mornings/baby classes you can go to. I'm very antisocial, but somehow found myself very comfortable getting involved and making small talk with strangers. You will find solace from other mums those going through exactly the same things as you.
Go to the Drs and ask about counselling. PND is so common and the NHS are really keen to catch and deal with it early. Your GP should have a drop in session with the HV. Talk to her about all your baby related concerns, that's what she's there for. I found this book incredibly helpful and reassuring.
www.amazon.co.uk/Your-Baby-Week-ultimate-caring/dp/0091910552/ref=sr_1_1?ie=UTF8&qid=1422369627&sr=8-1&keywords=your+baby+week+by+week
Take time to look after and pamper yourself. Long baths, nice food - have some planned time away from the baby as you need the break. Remember that you'll never be more tired and hormonal in your life. And cut yourself some slack.
Best of luck 