Well...... FWIW, does NOT have autism (although she has sensory issues), and the ENTIRE SUMMER basically involved me trying and failing to get Natalie to sleep and finally giving up and putting her in the carrier or the sling just so I could get something done besides sitting in her room trying to get her to sleep with Josie running down the hall, sticking her head in the door, and shouting BOO at the top of her lungs.

As a result, I've never gotten around to breaking the nurse-to-sleep association - sometimes I still have to, others I don't, depending on whether she's sick or teething or (Heaven forbid!

) hungry. On the other hand, doing that, I'm guaranteed about 5 minutes to put her down, and Josie is now OK with occupying herself that long, instead of endless 30-minute sob sessions with me AND the baby both in tears.

Do what you need to do to get by. You can try looking for the post "How I Survived the First Three Months" by Theosmom - anyone remember what forum that's on? - I found that it helped me a LOT to stop beating myself up about the stuff I tried that didn't work and the stuff I never got around to trying.
You might also want to try hiring a neighborhood teen for a break one or two afternoons - if not with your oldest DS, then with the baby, to just give you some relative down time. During the day, maybe you can find someone in your neighborhood who homeschools, who'd be at home during the day, for a regular break for yourself?
Good luck and hang in there!