Dear @Exhaustedmumof3dc, I am so sorry that you are having such an awful time with your DD at the moment. My only immediate thought is that I hope you are fast asleep right now (04.36am).
My second thought is, whether your own GP, and DD's GP, ate aware of the latest developments regarding your DD? I am hoping that you both have the same GP as each other, as that can really help with the overall picture for them, and could also affect the way any consequent guidance may be tailored for both you and DD.
I would expect them to consider your DD's recent relapse in behaviour as concerning, mainly because you, OP, do seem sadly to be too close to having an actual breakdown, which would, of course, not only be horrible for you, but also for all three of your DCren, and your husband, and maybe wider family as well. So, if for any reason your GP tries to say that her behaviour is totally normal, and will probably sort itself out in the following weeks, or months, please explain that you just can't go on for even a few more days, and certainly not for months, or even weeks.
Having said all of the above, I think that you might benefit a lot, if you can immediately - well probably tomorrow now, given the time of day it is at the moment - arrange to have at least one day and night, following into the late afternoon of the next day, away on your own, maybe staying at a Premier Inn, or even better, at a nice hotel with a swimming pool and gym attatched to it.
Of course, I don't know your personal circumstances, or your preferences, for having some good quality time on your own, but the hotel with a pool and a gym, along with a really good book to read, would be brilliant for me! Maybe you would prefer some time away, in say a cosy, old Inn, surrounded by stunning countryside, or even some time away in a boutique hotel in the middle of a city, one that has a lot of free, or inexpensive, cultural activities to enjoy?
If you could extend you stay away to at least 4 nights, that would give you time to have a nice, long, sleep, and maybe an indulgent, and lovely bath, with your favourite essential oils in it. Then, a day or two later, you could spend a couple of days spending your time doing exactly what you would love to do, if you didn't have you children following you around all day as if they were extra shadows!
Also, you having a little longer rest away from your exhausting daily life might be just what your baby needs as well?
Without you being in the house, your DH could show your youngest DD, every room in the house, to show her that you are not there, so if she calls for you, she will hopefully start to realise after a few circuits of your house with her DDaddy, that you are not there to answer her calls. I know that that sounds heartbreaking, but If your DH keeps on simply saying, with a very relaxed (acted?) voice and stance, that you have had to go away on a little work trip, but that you will be back on such and such a day, and that he is looking forward to doing all the things with her that she needs doing, including any that Mummy usually does.
Then if your DD begins to realise that she can manage without you, her DMummy, for a while, if she has her DDaddy, then - with everything crossed🤞- maybe she could start to turn a corner. I believe that he should also tell her (and your other DCren of course) that he loves her very much, and that, in fact, it is lovely for him to be having a few days off work in order to spend such a wonderful quality time with her (It doesn't matter if she doesn't understand any of the words, as his love, care, and pleasure should shine through the words), and that he is looking forward to them both having a fun few days!
I do really believe that you desperately need a break from your daily life, and I definitely think that it would be a bad idea for you to talk to your DD whilst you are away during that time, and hopefully Daddy won't mention you to her unless she says something about you first. If DD keeps on asking for you, he just needs to keep telling her that you will be back on whatever day it will be, and to remind her that her Mummy loves her so much too.
I understand that those few days you are away might be a nightmare for your DH, but he mustn't moan to you about it, he needs to really show what a great and considerate Dad and Husband he is! He also needs to learn as soon as possible that he is quite capable of being the main child carer for a few days, because if you got the flu, or say, had to be in hospital for a few days, you would all be relieved to know that your DH can manage perfectly well on his own for those few days. I can't decide whether it would be a good idea or not, for the other DCren to stay with their Grandparents for those few days, if they were willing to have them, as I can think of both positives and negatives in such a scenario?
I very much hope that you do ask any relevant authorities for advice, and any help that they can give. By the way, you come across as a very loving and caring DMummy to me, you certainly haven't failed at all! Good luck going forward OP.
😊🩷xxx8