It is a facet of this time of year, that most people feel a bit down. For some there is a condition known as SAD, one cause of which is a lack of Vitamin D ( daylight, eggs,oily fish or pills to fix) This may be you, Ex, DD and co-workers all
It is in the nature of teenagers. Just hold on and when she does it, just say "you know that I love you" If she is cross because you have not done something, point out that she is old enough to do it herself. As to ex tell him to back off - he is jointly responsible for his DD.
As to work, have you previously been the one that has taken up the slack and gone the extra mile, and not felt like/been able to recently? Either way do push back anything that is not your job and just do what is expected in your job description - if you cannot, then speak to your line manager to get help. If anyone moans, then simply tell them you are doing YOUR job and perhaps they might like to do the same. If it gets too bad, complain to your line manager/HR.
It may, of course, be a side effect of your mood/health, that you are just now noticing that they have always been shitty to you (ex aside as you did know that one).
Cuddle the dog, stop listening to Ex, speak only when necessary, stop pandering to DD (perhaps threaten them with each other?) , moan back at co-workers if you feel inclined, ignore them, look around at what jobs are out there. Polishing up your CV is a great way to a) realise how good you are b) plan for the future Please yourself a bit more, you are worth it!