Dear @Spikyseason
A few points just to contrast what others are saying and give you food for though.
Firstly, the necklace, whilst expensive, might not be a sign of love. It may have been an attempt to impress, particularly if she is very successful in her own rights.
Even if it was a sign of a much more serious relationship than he’s been honest about, it probably doesn’t negate the fact that he has a longer term, possibly deeper love for you, that he took for granted.
I’m not saying that’s a reason to forgive him…what’s he’s done is atrocious. But some of the comments here will make you doubt yourself and how much you are loved and that’s not fair either. Men can have strong feelings for more than one person at a time, which women don’t seem get.
The necklace sadly will act as a permanent reminder of the affair to you. But actually also to him. He knows the pain it will cause, which is why he wasn’t open about it from the start. Plus if he did buy it to impress, he realises now how stupid that will look to just about anyone else, yourself included. He has to live with having given that away. I think that will not be a proud thought.
So, whilst my gut feeling is probably LTB, because he’s a cheat. If what you want to do is save your marriage, then I don’t think you should compare yourself to the affair partner, because the necklace doesn’t equal love and neither do I think it says commitment. I think it makes him (how would my pupils put it) a beg?
He'd have to absolutely commit to being open, honest and transparent going forward. No more lies.
However humiliating, he’d have to explain his reasons for having the affair in the first instance.
If he still has money to spare, then he should spoil you with something worth let’s say £30,000.
That last one’s just for fun.
But ultimately, even after all this you may still decide that you deserve better, and that’s ok. You can choose to leave whenever you see fit. You don’t owe him anything and it’s his job to now put in the effort.
Im sorry this has happened to you. I hate to read people making comments about what he must think of you though, because none of us know him or you, and I don’t think strangers claiming he loves her more than you is helpful.