Late discovery of early betrayal

I've been with my partner for 2 years, we live together and I'm step mum to his 10 year old son. We've had a pretty difficult last 6 months since I moved in with him. We haven't found our balance yet, we argue, I do more than my fair share of housework becashe be works late, he doesn't prioritise me, among other things, I haven't been feeling loved. We had decided to have couples counselling to try to work on our issues, but hadn't started yet. Then BOOM!

After 2 years together, I checked his phone for the first time. I hated doing it, but something in me just couldn't resist. I found messages from a girl, who had the same name as me, from when we first started dating.

Give some context, in our first date, he asked me if I was seeing anybody else, I said no, he so said no, which turned out to be a lie. He told me he only wanted us to see each other and didn't understand how people can date more than one person at a time, and I agreed.

Fast forward two years, I find out he was seeing another girl after all. He had been dating her before our first date, continued to text her and arranged another date with her which was supposed to happen 2 days after we slept together for the first time. The date didn't happen because she called it off on the day, but he was checking with her that she was still wanting to meet, sending her messages with kisses and talking to her like he was very interested.

He was texting us at the same time, I imagine him sitting there sending one to her then one to me. He was even telling her about our second date, lying about the food he ate and acting like it was with his friends and not me.

I feel so betrayed and my brain can't even begin to start rewriting the history of our love story. This has shattered every memory I have and question whether any of it is even real.

He bought me flowers and I sent him a picture of the flowers, his reply was 'first of many', but turns out he had just been dumped a few hours before. He replied to her saying right girl wrong time.

He is making excuses which I can't even listen to, trying to minimise it, gaslight me by saying we had only been seeing each other a short while, he was winding it down etc. He's lying about it even now.

I left and am staying with my mum while I try to work through all of this in my head. It's bad timing as we're currently heaving a new kitchen fitted, so I've left him with no kitchen.

I need some help figuring out how to deal with this. Part of me thinks it was so long ago, should I even be that hurt, but I am hurt, I'm so hurt and my reality feels in question.