Hey lovely.
First and foremeost, is it like controlled crying? Absolutely not. No it isn't. You know me, I don't like controlled crying. I think it works because the baby learns that there's no point crying as opposed to them learning a complicated lesson beyond their grasp about sleep. I think in L's most formative years you did what I believe to be one of the most important things you can do and that was to teach her that when she needs you and calls out to you, you respond. She is a bright, well adjusted and secure little girl. You have done marvellously in the face of your own worries about your parenting and with an absolute lack of support and well meaning advice from every quarter (this one included).
Can I reassure you that the first time M slept in her own bed, both DH and I lay in bed with tears running down our faces. She's part of us, we adored having her right there with us, snuffling and shuffling and wrapping her arms round our necks at 3am. But the time came. Was it difficult? Yes Was it the right thing to do? Absolutely. Do we still have cuddles in bed? God, yes. Does she sometimes come in with us in the middle of the night? Yup. Is she afraid of her bed? No. Does she hate me? No. Has it hurt her? No.
It might not be about rapid return. We didn't have to do it though I think I probably would have been prepared to do it had the need arisen. The transition was fine. She understood, we changed the routine and stuck to it firmly. Bath, upstairs, put her doll to bed, choose 3 books and the books only get read while she is in bed and lying down. She moved or sat up, the story stopped. Books over, nightlight on, main light off. Happy, gentle, smiley bedtime, no cross faces, no worry.
Let me be utterly blunt. You cannot and will not do this on your own. DH must help you. He cannot witter on in your ear about it and then refuse to take responsibility and he can bugger off with his 'you created this situation', that doesn't matter because yes maybe you did take her into your bed but was anybody helping you at the time? Supporting you to find another way? Nope.
You must want to do it and you must want it to work. You have to accept that it will be sad for you but it doesn't have to be sad for her and it won't damage her, she won't hate you. I actually don't think having a newborn and a toddler in your bed is an insurmountable thing when everybody is happy. But I suspect that it would create tension on top of tension for you. You'll panic about it more and more, DH won't accept it and because you will have a newborn and he will absolutely need to be in with you, you don't want to crack and be moving L in haste and anger because nights are becoming unbearable. You can make this a lovely, kind transition for her and not all bound up in the change of a new baby turning up.
And not having somebody to cuddle at 3am. Oh lovely . You need to want to cuddle DH. Have that lovely time as husband and wife before your gorgeous girl crashes in, well slept and demanding huge cuddles. I'm worried that you don't want that? I'm worried that DH isn't doing enough to reassure you that he wants that.
It feels impossible to you now doesn't it? Oh it isn't. It's so simple. You love her, she loves you, she's mere feet away from you all night and it's done. That could be you in mere days. You just have to want it. Don't decide that it's going to be terrible before you start it. Tell yourself how smoothly it could go, smile and embrace it and if it gets tough then fall back on a plan.
I've got to go out now, sorry. I'll be back later to waffle more.