@JulianCasa I'm sorry you're dreading tomorrow 😔 it sounds like a tough situation and I have had similar (albeit secondary, as a HoD) so I know how hard it can be when you are teaching your classes and also planning for others plus supporting covering/new staff.
I hope that you can do something nice today, even just for an hour or so.
More practically speaking, for today, just focus on what is absolutely essential for tomorrow. Are you planning for all 3 classes? Or is ECT planning their own? Is it possible, going forward for you and ECT to share planning to ease the load?
I saw a quote recently that struck a chord, it was along the lines of ' when you feel anxious, remember, one thing at a time, one hour at a time, one day at a time'. May be helpful to think that way for tomorrow.
Longer term, can you speak to the Head/another SLT member? It's not on for you to be shouldering all of the responsibilities- I would explain that while you are happy to help, you cannot do it all and ask how they can support you with the workload.
It's really tough at the moment, remember to be kind to yourself!