All I can say is what everyone else here has said: that first year is hard. Seriously, seriously hard. So much of what you have said rings enormous bells with me - I just feel so resentful at times that his life hasn't really changed whereas I feel like I don't have one anymore in particular.
We used to have arguments about how much of his book he had managed to read on the train on the way home 
I remember those 10 months of maternity leave just being so very, very weird for me. Being at home all day every day just wasn't for me. I loved - love - my DD more than anything but staying at home, going to baby groups, all that wasn't me.
It's perfectly alright to think like this, and also to think that maybe returning to work is what you need to feel right again.
I can only echo what other people have said. This has been a massive change to your life and it takes a long time to adjust. Things can get very stressful, but it doesn't mean that your relationship is in trouble. You just need to slowly settle into how much things have changed. That may be you returning to work, it may be you as a couple gradually managing to get the odd night out together (though of course there is that conflict the first few times between wanting to be out, and then when out just worrying about the baby)
I found the biggest help, though it felt quite alien for me to do it, was to keep being brutally honest about how I felt and to try and keep explaining it to DP. That whole "your life hasn't changed as much as mine has" line. Don't say it accusingly; there may very well be nothing much they can do about that.
And most of all, try not to view it all in an overly pessimistic light. It is easy to put a negative spin on things and make it worse. Yes, this is tough. Yes, one or both of you is still having difficulties adjusting. Yes, the lack of couple time can vastly dent your relationship. Yes, things are not a bed of roses. But that doesn't have to mean that things are all rubbish. It's a difficult phase. All relationships have their good times and their bad times, and there is nothing like a young, demanding baby and a complete role reversal in one partner's life to start a bad time.
As your DS gets older things will get easier.