The other thing to remember op is that the anxiety is exacerbated at this parenting stage because you are no longer in control. You can advise and support from a distance but after that it's down to your adult child to deal with the consequences of their mistakes. And it's really, really hard standing back and seeing them make those mistakes in the first place.
Ditto your ex getting engaged. It's not your job to deal with all of the fallout.
It's so hard with your mum's diagnosis too. Another thing which is outside of your control.
If you can access some talking therapy via your GP to support you through this difficult period, that would be great.
If you can't though, then I think you need to take a breath, step back, and try and focus on your own well-being. Try and remove yourself a little emotionally. It's very hard but you can't support anyone if you are not in good shape yourself. So ring fence a bit of time for yourself to relax and do things that being you peace and enjoyment. And focus on good sleep, a healthy diet and a daily walk if you can.
Also, remember that just by being there, and listening, and supporting, even if it's in the background, you ARE helping!