You need to talk to someone about it. What you've done today is definitely a good start, admitting that perhaps things aren't quite right for you.
If your partner is supportive perhaps talk to him first. Tell him what you've written down here, even show him your post.
There is nothing wrong with going to the doctor and having a chat about how you are feeling. Meds aren't for everyone, but it's worth talking your options out with your doctor.
The most important thing is that you need to talk it out. Start taking up those offers of help. Even if its someone coming over and folding your washing! Or coming over and having a cup of tea.
Being a mum can be a lonely job. For me I find the mornings when Cooper is asleep the hardest. I also find myself craving for adult company, so I'll go out and have a coffee with a friend or even just got out somewhere.
Take care of yourself, that is the most important thing.