Your daughter has been assessed by professionals- both her Dr and by her teachers- as not suffering from anxiety or depression. You have involved the school who have tried various strategies to support her. You have done everything reasonable and within your power to support her to continue in education. Despite this, she chooses not to attend school, but will not consider alternatives such as finding a job or an apprenticeship. Instead she lounges around in bed and hanging out with her friends. It also sounds like she is rude to you, disrupts the household and is not pleasant to be around. I doubt she does much to help with household chores, cooking etc- I bet you cook, clean, do her laundry for her? And bankroll her living expenses, phone and socialising etc?
Exactly what incentive do you think your daughter has to either go to school or get a job? I know you think this is a reaction to the breakdown of your marriage and covid- and perhaps in some part it is- but you can’t let her hold you to ransom in this way. None of those things are your fault, so don’t feel guilty about them. And definitely don’t let her (intentionally or not) use it as a stick to beat you with. It’s doing you no good, nor your other children- but actually by giving in and letting her away with murder, you aren’t doing her any favours either.
I am afraid I would be very blunt with her- she is 18 years old now and needs to start taking some responsibility for herself. If she’s old enough to make these sorts of decisions (which she is), then she’s old enough to bear the consequences in their entirety. It’s not as though you haven’t sought help and support for her, you’ve tried hard and it sounds like the school has tried hard to support her too. I’m not quite sure what you mean by “3rd level”- is that university? I don’t think she has thought that through- her school won’t be able to give her a good reference if she wants to go to university- and will she even get the grades if she is not attending? I think you need to tell her that she is going to have to take responsibility for herself- she either gets herself back to school with the support they have offered, or leaves school and goes to college, starts an apprenticeship or gets a job. She can always go back to education in a few years if she wants to. Her choice, but a she has to do something.
If she won’t, then I would stop paying for anything but essentials- she gets a roof over her head, food on the table, essential toiletries (no luxury brands- basic toothbrush/toothpaste, soap, shampoo and conditioner, deodorant) and that’s it. No TV, no Wi-Fi password, no paying for phones and clothes (other than bear minimum when something needs replacing), no bankrolling the costs of socialising etc. If she wants those things she earns and contributes to the household. In addition, she starts taking on some of the household chores. If she doesn’t like these rules, then she is free to leave and go to her father’s or to a place of her own. You aren’t throwing her out, just stating the rules of living in your house and being fully financially supported in doing so.
I have no doubt she will be angry, possibly tantrum and try to guilt trip you into backing down- don’t. You are actually helping her by making her be a responsible young person, not a lazy layabout. She won’t thank you now, but she will in the future if you can help her turn things around.
As for yourself- I think you need some support in all this. It sounds like your mood isn’t great and you might be a bit depressed. You shouldn’t have to be at home 24/7 or end a relationship that made you happy, or stop seeing your friends. That is not healthy.