To ask how you switch off from work (part time) or do I need to say something here about workload.

[ItsAllBlahBlahBlah]

Just struggling abit after rtw from maternity. I've been back seven weeks or so now and had been off for a long time due to sickness at the end of my pregnancy. Oh and I've changed jobs (!), although in the same wider team and still have the same manager. I'm a manager myself but only one direct report who is also brand new to this team. I currently working three days pw but my predecessor did four days but was also inclined to work through the evenings quite often which I don't feel I can do with two kids 3 and under. My manager has been quite supportive when offering me the new role and said all the right things, that the role would be tailored as they don't expect me to be able to attend all of the meetings as my predecessor etc but after days like yesterday I feel like I'm already disappointing them. I've now logged off for the weekend but can't switch my brain off from worrying about next week.
I don't know what to do as I can't afford not to do this job, I'm not sure anyone would be so flexible and pay the same (it's not a huge sum but it's fair and rare they've been so flexible). I don't want to go to four days either. I feel if I say something I will be talking myself out of a job but think somehow if I don't they will end up just criticising the fact I'm not performing the same as my predecessor. I also have a habit of worrying too much so with lack of sleep and trying to catch up on training myself i wonder if I'm beating myself up for something I shouldn't be.

Is anyone else part time and can offer some advice how to switch off and not drag work through the weekend.

I went back part time after maternity leave in 2014 and continued after second child. I work 24 hours a week over three days. It's a huge adjustment at the start and yes, the meeting thing is awful. I'm a senior manager and very lucky that my workplace is so supportive.
Things that help;
Start blocking in 'focus time' on your calendar - mark it as busy so people can't book meetings in and then plan that time for yourself to complete work.
Speak to your manager about priorities and be realistic about what you can achieve.
You have to let some stuff go and let other people take it, even if it's something you really want to do
Have clear boundaries. I do often keep an eye on emails and my team know they can contact me with urgent stuff but that's my choice and I do say no to stuff I know will be stressful outside my working hours unless it's critical.
Whilst your company might have a culture of working extra hours, remember that proportionally, it's very easy to work way more if you're part time and you're not being paid for it (eg if you work 22 hours, an extra 7 hours a week is a third on top, if you're full time it's a fifth).

@Fluffandbubbles is right about using your out of office.

I work 17 hours over 3 days. My OoO goes on when I knock off on a Thursday and tells everyone that I'm not back in until Tuesday.

It's reduced people's expectations of an early response, and anything urgent now gets resent to another member of the team.