So when do you get to sit down and have a beer (or your equivalent) OP? When was the last time you actually had some time for YOU?
As I said earlier, get yourself back to the doctor, there are lots of other anti-depressants other than Sertraline, so tell the doctor how it made you feel, and ask if they can give you something else.
Moving on, is your partner easy to get along with, or is he the sort of man that's happy to sit back and let you do everything, but will then blow up if you tell him you're struggling?
If he's easy to talk to, then I suggest that you sit down with him, and tell him how you're feeling, not just that you're feeling a bit down, (we women have a tendency to down play things) but that you feel that as a couple, you have fallen into a pattern of doing things, and it's now causing you to feel unappreciated, overwhelmed, and taken for granted. You could say that you realise that it was never his intention to make you feel this way, but it's the way you're feeling, and something has to change, otherwise you'll eventually break down both physically, and mentally, and be of no use to either your child or him. Explain to him that while you're fully aware that he works too, but what you do is not a 9 - 5 job, nor a 6am to 7pm type job, you are literally 'on call' for 24 hours, and you need some help. If he can't or won't pull his weight, or even give you a break, tell him that he'll have to find some extra money to pay for it then, because quite simply you CAN NOT do it ALL!!
However, if you feel you can't broach this in a simple, calm way, or he's the type to shout you down if he feels attacked, then try writing it down, that way he has no opportunity to interrupt you, or break your train of thought, and it will give you the opportunity to get your thoughts across.
Please tackle both him and seeing the doctor as soon as you possibly can OP, as things will only continue to deteriorate, the longer you leave it.
Good luck, and sending you a big appreciative hug, for all that you're doing for your family.