I think it's incredibly common to feel shattered/emotional/depressed at this stage.. Myself (dd 14mo) and friends all seem to have gone through a rollercoaster of a time last few months whether we sahm or working (all of us v.similar age to you)
I think what I'm finding is things change considerably when baby becomes toddler is what I am finding anyway, as you say, before I could enjoy a leisurely coffee / lunch with friends - no chance now, I used to enjoy trips on tube as could sit read a book whilst baby slept or played contently with a toy now it's a 'battle' to keep active madam in her pushchair !
Do you get time together as a couple ? meals / drinks out ? I feel that's really lacking in mine and dh's relationship at moment as we don't have willing supply of babysitters local to us..
Perhaps you could look at part time work ? I don't think as others say that you should beat yourself up and feel you must be 'grateful' Jeepers, I love my days at work - it's a break !!
The only other thing I can suggest too is actually I think slightly contradictory to what other's may have said - I think it could be fun to have a 'girls' night out with old work colleagues, single friends or just longstanding friends - whilst don't get me wrong I enjoy spending time with my 'new' NCT friends etc, it's not the same as a bottle of wine and a good old natter with an old friend, try and make time to catch up with friends you haven't recently seen maybe because of 'being a mum' - I found it was really lovely to just talk about totally different things and forget all about being a mum !
Hope all the posts help you feel saner - you are not alone !