Ydd was like this until just recently. We held her for every nap and she wouldn't stay asleep by herself for long. We slowly increased the naps we laid her down for while continuing to hold her for at least one long nap.
While she still gets very upset to wake at night past 1am by herself (it's like she's got a clock) she has recently started "asking" to be put in her crib and going to sleep on her own. It was like a switch. One day 2 weeks ago I sat rocking her to sleep she climbed off my