I feel like my life is over

Social services are only no help because you're propping up the situation. They would assess her as needing more if no one helped out. Which is a difficult choice you have, but a choice nevertheless. Prioritise yourself and your DD. In the prime of your lives you both matter more.
Inform social services and the GP you are all stepping away within 72 hours and if she phones you, you leave a message for them, each and every time.
In a cruel way you want things to get worse for her otherwise nothing will change.

Please get a further assessment for your ex mil. I cared for both my parents, one for 3 1/2 years in my home. I am one of four and I had no help from my siblings. We all suffered including my 18 year old son who helped me. My son had to do my late father's personal care (carers rocked up at 11.30 for his breakfast call! ). You know ss will let you do it all. He loved his granddad but it was too much with his college work.
I was left with a career in tatters (I had to resign after my father had a particularly bad fall). I also had mh problems and an alcohol dependancy. It has taken my 5 years and a six month course of private therapy to begin to feel better. My counsellor likened it to PTSD.
I am afraid I would never do it again. Please get some help. Anxiety medication can litteraly save your life. I know a number of people for whom it has worked.

It took me two years to really recover from my Mum’s death, OP, and you’ve suffered three bereavements close together. 💐. Give yourself time, you may need several years to truly recover and don't rule out further counseling and medication.

I was later diagnosed with GAD and I’m on AD’s ( very low dose). No side effects, I just don’t get quite so anxious and I cope much better with whatever life throws at me. Medication may also help you get through these difficult times and you can always stop taking it if you wish.

I'm appalled that a 19-year old girl is going to be sacrificed to an elderly person ☹️
Hopefully she will flee as soon as she realises what is actually involved

I should say - I had a family bereavement and have a terminally ill parent so I do understand how awful it all is. I was on meds for a while and now my self care routine involves taking time out for exercise and things I enjoy. But you're absolutely not going to feel better in yourself if you and DD keep getting landed with caring roles - it's someone else's go this time.

Your DD shouldn't have this level of responsibility at this point in her life - she's only just learning to be an adult herself.

She needs to tell SW that they can't use her as a contact. The more she does the less SW will do. Blocking numbers so she can call her gran when it suits her but her gran can't use her as an on call emergency helpline is not unreasonable.

Also, you say "If we all said NO not doing it and refused the practical care the worry over exMIL wellbeing would still be there and worse for her."

If she won't do it then it will push things to crisis point and other people and agencies will have to step up. I'm not sure how this would be worse for your DD than having to sacrifice her freedom as a young person and very, very likely get a worse mark in her degree to look after her granny when you say she's already struggling and "dreading it".

Your DD needs to give herself the best shot possible at the rest of her life and she's not going to be able to do that if she's pushed into a caring role.

Why does your DD feel this responsibility?
Is she trying to protect you? You can’t just say oh she wants to do it. She must feel some obligation from somewhere. I suspect from you (albeit you have not communicated this directly)
Get outside help.

You are all finding the future difficult, maybe its time for your mils to live somewhere more suitable for their future, your 19yo dd should not have to worry about this, is it time now to start looking for a carehome instead.

I am sorry for your losses. I've been bereaved twice in the past 2 years and I know it's hard.

I also have a 19 year old at uni and she hasn't been able to help with any care for relatives because she's a distance away and for that I'm glad. I agree with everyone who says you can't let her do it but I get what you're saying when you point out that nobody cares if you're coping.

But you're not coping. Your dd won't cope. It's unfair. I think your ex DH needs to sort it out and needs to step up and his MH can't be a reason, your mh is suffering here too and it's not even your parent.

They need another number to call and it can't be your dd. I'm so sorry, I don't know what else to suggest but agree that grief can't be rushed and that a combination of counselling and medication might be worth exploring. Good luck.

My 3 Aunties does within 6 weeks of each other and they were like second Mums to me and my sister's having no children of their own. Then within 7 months my Mum died of a horrible pancreatic cancer that my sister's and I cared for Mum for last 3 months of her life. It was brutal and traumatising. Loosing so many loved ones in close proximity left me numb. It took about 3 years before the worst of the sadness left me. I still dream of them often now a further 6 years later. Grief takes time. Nothing can make it pass more quickly. In my opinion grief counseling is better after a year or 2 as your mind is more open to it then. Ring SS and tell them there is no one available to care for ex mil. Don't allow your dd to sacrifice herself. She should go to see her Nan and chat to her or make her a hot drink but not be one her carer.

Sorry op but there's absolutely no way I would allow my teenage dd to become a carer for anyone.

I've considered talking to the GP but feel reluctant to take meds. Maybe I should discuss the merits. What meds will do is lift you off rock bottom enough have the motivation to do all the other things which will help you get back to normal

Does the grandma need to go to the next level of care, supported living may no longer be suitable for her

Family need to completely step back here and if these family members end up in a home after a crisis then so be it. No one needs to step up and sacrifice their own mental well being. Different priorities need to be made, hard as it is in the short term.
Surely these elderly relatives when of sound mind wouldn't have wanted this life for their busy adult DC with jobs and families of their own and definitely not their GC. Would you expect your DD to commit to caring for you indefinitely in the prime of her life? That's what our assets are for.
Dreadful.

It's not fair on anyone unless they actually want to do it.

what are the other options OP?

Please do consider Counselling and Medication.

You really do need to take the first step and talk to your GP. Only they can talk to you about the possible side effects and benefits.

Is there a grief charity near to you? We have one that offers groups sessions and grief counselling.

But please reach out and do something.

Why is your DD doing it, where is her father and his family (assuming he’s your ex but can’t work it out)?

Also, how old are these ladies and how advanced is there dementia?

I absolutely would not allow my 19 year old to become a career if it could be helped. Especially not for dementia. How has this happened? And what are the circumstances around your current MIL?

i feel your pain, it is exhausting.

@DahliaMacNamara Agreed!

Your DD is an adult and obviously free to make her own decisions. That said, I have an IL with advanced dementia and my DD is the same age as yours, and I would be seriously discouraging this at any level beyond maybe occasionally going along with you when it's your turn to deliver care. She shouldn't feel responsible for any of it.

I’m another one who agrees that a 19-year-old shouldn’t be taking on this level of responsibility. Where are her Grandma’s children? How can they let your student DD sacrifice her youth looking after their mother, it’s totally unfair. Please push them to step up.

My DD is 17 and I’d be incensed if her aunts and uncles allowed her to do significant caring for my MIL. I’d tell them to get off their backsides and sort things out themselves. 😡

Why is 19yo going to be main carer?

Totally agree with others that a 19 year old should not be allowed to take on this responsibility.