sleep training - naps

My 6 mo old daughter has started this pattern of being unable or unwilling to go to sleep on her own, e.g. She must be rocked to sleep, only to wake wide up as soon as her head hits the mattress. This happens at nighttime and nap time. I've tried a few different things to help transition her (holding her until she's drifting off but not asleep and putting her down, cry it out - her will power is stronger than mine with this, and comforting her without picking her up - this seems to actually make the crying worse). My biggest thing is i don't really get how to put her down for a nap when she won't even be put down for bedtime. Once she does fall asleep at night she's usually sleeps through the night, but getting there is a nightly struggle. Anyone have any tips? Also, is it possible the whole not sleeping well thing is a phase? Because she used to sleep so well.