I wouldn't put her back in nappies. She's proven herself to be capable of using the toilet and doesn't have an abnormal problem with wetting herself, considering she's only been dry a few weeks.
I wouldn't worry too much about her holding in the urine all day. Instead, I would nonchalantly place her on the toilet (never ask, always tell!), saying "time to sit on the toilet to see if we can let out any wee's this time". And talk excitedly about how clever it is to let the wee's out into the proper place they should go, that sort of thing. When I was potty training mine, it always came with a treat until it was well and truly established.
If, after a long time she hasn't peed, then just say "oh well, never mind, maybe the next time, good girl for trying. You can't have a sweet this time but next time, if you let a wee out, even a tiny little wee, you can have a lovely sweet". Or treat of a more healthy option!
And the hard bit? Let it go. Act as if it really honestly doesn't matter. Pretend if you have to.
2 hours later try again. And ply her with drinks galore. Nature will win in the end!
Your relaxed appearance will help relieve any tension she might be picking up from you.
But I'd also say, if she's reacting to a stress in her life, you need to find out what it is and deal with it. So if there's a problem with her daddy then you can't ignore that.
I found going to soft play of toddlers groups was great because I could hang around and eventually do so from a distance. In the end my DC would ditch me at the door to find his favourite toys. At that stage she will be feeling safer without you.
She won't always be clingy. I remember studying child development in uni and psychologists with attachment theories suggest a child who is made to feel safe doesn't need to cling, but a child who is clingy needs to be made to feel safe. The more she feels safe around you the more he will feel confident to explore beyond you.
I appreciate your problem with needing rest though. I suspect if you can have a helping hand for a while, rather than a complete break for so long, you might see an improvement sooner than you dare hope. And then you've got a potty trained hold happy to go away from her Mum for a bit.