I would totally second DJ's Mom. Carry your baby in a sling, particularly during the fussy times and times you need to be somewhere. That way, the baby can nurse and snuggle while you interact with your older child and get stuff done. If you aren't already, is there any way you can cosleep so you can at least nurse laying down? Once I figured out how to do that, I felt like a new woman!
A mother's helper, if you can get one, would be second on my list of things to find. You could clean, or rest, or whatever, while they play with at least your three year old.
And you will get through a year. My SIL has six children, one born during her hubby's first tour through Iraq, stationed out in Hawaii, thousands of miles from any family or friends. She wanted to tear her hair at times, but she and all the kids survived and thrived, actually. He's been redeployed back to Iraq again, and so far, so good.
By the time your DH has to leave in January, your baby will be much easier to cope with and not so high maintenance. And your three year old will change too in the next few months. It's so hard to remember how quickly they change when you are in the trenches.
Hang in there! We ALL feel like non-successes on some days.
Oh, and formula is a big hassle. You have to a) get out of the house and go buy it, a feat with two small children, b) wash bottles and all that jazz, and c) then contend with your engorgement. It could damage your supply in the end. It also costs a small fortune. You can make it through without formula