There have been so many great ideas on this list that I feel inadequate in adding my two cents. While you clearly need a 'helping hand', it's hard to find one when you are so exhausted all the the time. Even a teen for a couple hours after school or on the weekend might give you a bit of breathing room so that you can focus on yourself for a few minutes.
But the big thing to remember is actually a little thing: "This too shall pass!"
Don't feel guilty about wallowing in a little self pity now and again. You have a tremendous 24/7 job with no breaks! If you are feeling depressed and overwhelmed, remember that this is a temporary situation. Kids don't remain toddlers and babies forever. It's a horribly long time when you are sick and tired and in the middle of it, but give yourself a break. So, the house is disorganized for 5 more years. Do the best you can, but remember that in 5 years you will only have one child at home during the day and you can do more then.
And remember that just because you feel tired, unmotivated, disorganized, etc. that this is not YOU this is not who you are... it is simply a temporary state of being.
I really, really felt like I had lost myself when my kids were younger. I had no hobbies, did nothing that I didn't absolutely HAVE to do, resented anything 'extra' that came my way. Overwhelmed? You bet.
Do you know, I just completed my SCUBA Open Water Diver certificate! If you had told me 5 years ago that I would do this, I would have laughed out loud in your face! Hah! We're broke, I have a special needs kid, I have no time and no desire to do such a thing, and I'm fat and out of shape. I'm a mom. Then I'll be a grandma. Then I'll die. [Yup! I was the queen of the bad attitude!]
Then, one day, I dropped the kids off at school. I took a nap (!), woke up refreshed (!!!!!) and suddenly discovered that I have a personality! Where did THAT come from?
Sure, I still get depressed once in a while - some situation, some physical - and I pull out the age old mantra "This too shall pass"