Also--here's a reason to talk to some managers.
You say "returning to my previous role is not a possibility...." but I think it's quite frequent that people think something is impossible when it really isn't.
So here's my other suggestion: Call your OLD manager and ask them to go to lunch, or something. Partly so you can seek some advice from them about how to handle this no-training situation, but mostly so you can spend some time in the company of someone who has seen you be competent and comfortable at work. So you can reconnect to that person.
But also because I think you need someone in management to help you test out the idea of whether it's at all possible to change your secondment. I'm not in your country, I don't think (we don't use that term in the U.S., and maternity leaves are seldom 12 months), so I don't know the norms. But I think you should find someone at a higher level than you who can help you probe that assumption.
I had a situation awhile ago w/ a boss you seemed to assume that I was always just wrong. Our communication styles were different, and I couldn't communicate in a way that made her see me as competent. Even though, at the end of any particular situation, it turned out that I'd done exactly what she wanted me to do.
I also felt very isolated; I tried to reach out to people from previous work experiences and kept getting blown off. When I got a new job and my boss and colleagues acted like I was competent, I can't tell you what a difference it made.
So I vote that you find ways to reconnect w/ people you were around back when you felt competent. (because you ARE competent) Bolster your sense of value there.
And also--start making a list of what you do right. Every day, start a new piece of paper and write down briefly what you just did that you did right; just two words. "Sorted out Johnson contract" (even if you didn't know what was going on w/ that contract at first, you sorted it out, and that's competence)
In your other thread, you talked about some parts of a contract not adding up, and not knowing what to do.
I want to point out this: knowing that something didn't add up is an achievement--that's competence.
So maybe becoming your own cheering section can help you buy yourself some time, and some emotional/mental well-being, while you sort out whether there truly is any way to change your assignment.
Sending you some big hugs--it sounds really tough.