Hi op, I'm another one this happened to and i really understand how you are feeling right now as I was there myself.
My husband (married 13 years, together 20, 2 children) had an affair with a younger woman from work. The difference is that I found out and asked him to leave. Not because I didn't love him, far from it, but because I was not going to compete or beg for someone to love me.
He stayed with her, mostly because he had no-one else left (most of his friends were our mutual friends and they wanted nothing to do with him) and they split up less than 2 years later. I on't know the full extent as to why but from what i heard, she didn't like the amount of time he spent with his children, her being 27 to his 40 and child-free.
By this point I had met my now partner. He is now with someone else who seems much nicer and has children of her own and we get on pretty well. It took a lot of work on my part to get here but I put all my efforts into our children and making sure they were as ok as they could be. It's a massive cliche and not really helpful now, I know, but time does heal. I still feel desperately sad that our marriage ended, especially the way it did...it didn't have to be that way...but I am in a different place to 3 years ago.
If i'm honest, I think he regrets what he did and where he has ended up. That makes me sad but I will never be someone's second best or fallback. I deserve more than that and so you do. Once you realise that you will find the strength you need.