Well, I'm with you in that DS HAD to be held, and I just can't do that now with DD (which yes, I feel guilty about). (I'd love to sling all the time, but after awhile my muscles need a rest.) My solution is to just keep trying with her. After a half-hour in the sling, or fallilng asleep at the boob, I try to put her in the cosleeper or the swing. If it doesn't work, I walk her back to sleep and try again in ten minutes. She slept from 10:30 to 2 in her cosleeper today. I cannot tell you how miraculous/mindblowing that is to me. Holy cow. I honestly didn't realize it was possible. I haven't worried about her falling asleep on her own yet.
Anyway, it does seem like I've gotten a kid who likes to sleep more than DS. And I am thanking my lucky stars for that.
Momma to 8 y.o. DS and 5 y.o. DD. Married to a Maker!