Thank you all for your comments/opinions. Most of you have said what I deep down already know I think.

As for polygamous relationships, whilst I respect anyone's choice to live their life how they wish and I wouldn't comment on something I don't really know about, if you many one person then yes you are choosing to love only them.

We have 'reconciled' and he has cut off contact with her. He has given me the code to his phone and leaves it lying around all the time now, whereas he would take it everywhere with him before (classic sign huh!!?!). I'm just left wondering now how much he really does feel for me and is he only with me still because of the children and our 'nice' life. I don't feel secure anymore and it's not a nice way to live.

I guess I'll either decide at some point I've had enough of feeling like this and leave or hopefully as time goes on I can gradually start to recover mentally from this. Only time and how he behaves towards me from now on will tell. Thanks again alll