Oh OP, I really feel for you. But please take a deep breath and try to consider your future. 8 years, a house and a dog seems like a lot of commitment but have you heard of the sunk costs fallacy?
It’s where a person feels they’ve invested so much time/effort/energy/money in something that they just can’t give up on it. So they keep investing in the person/relationship/situation and on and on it continues. But ultimately the thing they’re investing in isn’t worth it - the person should really have walked away the first time it became clear the ‘thing’ wasn’t right. But because they didn’t want to lose what they’d already put into it they just carried on and ended up losing even more years of their life and more of their resources in the long run.
Which is a long-winded way of saying that your partner is romantically pursuing another woman while you’re planning a wedding - so run, don’t walk! If it’s like this now, what will it be like ten years down the line with a couple of DC thrown in? A million times worse, I expect.
I was married to my ex-husband for 12 years when I discovered he’d been cheating, we’d been together 18 years and had DC, house, dog, the works. I invited him to leave. I cannot forgive the man for pissing all over our marriage and our family. He said he knew I’d never forgive him - so there was no point attempting marriage counselling - and he was correct. I don’t hold a grudge against him or wish him any harm - I just want nothing to do with him as he makes my skin crawl.
You have to decide how you want to live but I knew I would never be happy living with a man I couldn’t and didn’t trust. I had no intention of constantly looking over my shoulder, worrying about where he was, checking up on him etc. It’s just not worth it - a ‘relationship’ like that isn’t one that’s worth having, in my opinion.
I read somewhere that trust is like a glass bottle - once it’s smashed you can’t just put it back together and refill it.
I’ve heard stories about people working through infidelity and coming out the other side, but honestly OP, I can’t see how it’s worth it. He’s a slimy, lying man that you’re not legally tied to yet. Take this as your ‘get out of jail free’ card and get rid of him. He is proven himself to be not trustworthy - literally, he as a person is not worth and doesn’t deserve your trust.
I know you’re heartbroken and devastated - I empathise, having been there. But underneath all of your pain and upset you must know that you are worth so, so, so much more than what this poor specimen of a man has to offer. Somewhere you might find a sense of anger that’s you’ve been treated so poorly and a steely resolve to move forward with your life.
I wish I could you a big hug and tell you everything will be alright - because I promise it will. In a few years time you’ll look back on this as just something that happened, an upsetting time that passed. And I predict you’ll wonder what you saw in him and thank your lucky stars you didn’t stay. Don’t waste any more of your precious years on him. Houses can be sold/sorted - please don’t stay in a bad situation just because the thought of change is so scary. Things happen, people let us down, plans change.
When I was terribly upset about my ex-husband cheating a very elderly aunt (who has lived through a lot and is very sensible) said “Well, it’s happened. You can’t change that. But what you can control is how you get on with the rest of your life. Don’t let it define you.”
I took that advice and used it as a code to live by during the difficult days. 3 years on, life is good - I’m much happier than I was during my marriage. So I know there’s much better things to come for you OP - but only if you get rid of the dead weight. (Sorry this turned out so long!)