I was in a similar place about 18 months ago with a digital agency I founded and ended up selling it about a year ago. I don't regret the selling part but I know now that I could have made my own life a load easier if I just stopped and looked at things objectively.
It sounds to me like you need to get some perspective on things so I would suggest:
1) Talk to your co-founder. They probably see you in ways you don't realise and they should understand that for the sake of the startup it is best to give you a bit of support. If they don't then seriously think about whether you want to continue with someone who won't support you (and the startup) when you need them to.
2) Take a break. By this I mean a real break where your cell phone is off for at least a week if not two. Where you do as little as possible and try not to even think about work. It might take a bit of time to get yourself a chance to do this but you have to.
3) Learn to meditate. It will totally change the way you look at things. The feeling of suffering and pain you have comes from your mind being out of control - you know rationally that it isn't as bad as it seems and said as much above. Learning to calm your mind and get rid of the fog will help you look at what really is important and you will feel more in control and positive.
3) Remember that you are only human. There is nothing wrong with making mistakes and things getting on top of you. You must learn to not be perfect and be happy that you are as you are. There is no shame in needing help.
5) If the above don't work for you go and see a doctor. Depression is a real and sometimes very severe illness. For some people (but only some) medication is the right answer.
It sounds to me like you need to get some perspective on things so I would suggest:
1) Talk to your co-founder. They probably see you in ways you don't realise and they should understand that for the sake of the startup it is best to give you a bit of support. If they don't then seriously think about whether you want to continue with someone who won't support you (and the startup) when you need them to.
2) Take a break. By this I mean a real break where your cell phone is off for at least a week if not two. Where you do as little as possible and try not to even think about work. It might take a bit of time to get yourself a chance to do this but you have to.
3) Learn to meditate. It will totally change the way you look at things. The feeling of suffering and pain you have comes from your mind being out of control - you know rationally that it isn't as bad as it seems and said as much above. Learning to calm your mind and get rid of the fog will help you look at what really is important and you will feel more in control and positive.
3) Remember that you are only human. There is nothing wrong with making mistakes and things getting on top of you. You must learn to not be perfect and be happy that you are as you are. There is no shame in needing help.
5) If the above don't work for you go and see a doctor. Depression is a real and sometimes very severe illness. For some people (but only some) medication is the right answer.
Hope that helps.