I once dated a guy for 2 years. We had many conversations about our future, the breakdown of his marriage 9 years earlier and so on. We were at the point of discussing arrangements for me to sell my house and move in with him.
He was very slippery about whether we'd get married and so I was holding back putting mine on the market because I didn't think we were quite on the same page, and also I'd have preferred we bought somewhere new together but he was resistant and wanted me to move to his house.
At this point his mother let slip that "as of 3 years earlier he hadn't been divorced, but she was sure that if he had led me to believe he was divorced then he was by now". I asked him and he wasn't - they'd split up, made a financial arrangement, and never actually got divorced.
He was going to let me sell my house and move in, and his ex was his next of kin and would have got everything and been his medical next of kin if something happened to him!
Not only that, but his parents had recently given him a lot of money to fix his house up, and his ex would have got everything of that too!
He said he'd never told me because he knew I wouldn't like to date someone who was still legally married. And to him it was just a piece of admin.
He did then put in for the divorce, but a few months later when it finally came through, he didn't tell me that either, until I asked and he said 'Oh yes that came through a couple of weeks ago'.
I tried to step over it but he never apologised for the deceit, and kept maintaining he hadn't lied because he had never actually said he was legally divorced, and that letting me believe he was divorced and choosing not to tell me otherwise at multiple opportunities wasn't actually a lie. I never trusted him again and ended things, it took a while and I tried to step over it but couldn't, because if he could intentionally deceive me, then how could I ever trust him.
I was very clear he had made a fresh choice to deceive me every single time we discussed our future and his marriage breakup, and he'd been prepared to leave me legally and financially exposed.
OP, I think it depends whether your partner thought he'd told you - a PP said she thought that was something she'd told her new partner but they were living their lives, it wasn't a big deal and had never come up.
Was your engagement was with serious intent to get married soon, or whether it was one of those 'We got engaged just to show it's serious, we might get married in 10 years' type things? I've seen plenty of people get engaged before one is divorced, just to show future intent.
If your partner actually intentionally didn't tell you that would be unforgivable and I don't see how you could, or should, trust him again.
It's a huge shock and you don't have to make any major decisions right now.