I know it must be really hard to let her cry on her own, but if she wants to be alone and is going back to sleep on her own, well that's a bit of a good thing, isn't it?
Is there something that upsets her routine on those days when her nap is shortened? Is she OT those days?
I can only imagine how hard it is and how heartbreaking it must be, but I had=ve to admit that I am impressed that she will fall back asleep on her own and wants to be left alone. Does she take a lovey to bed with her to comfort her?