Why doesn’t it get easier?

I was with my kids father for 10 years. We split almost 2 years ago because he cheated on me. He is with the girl he cheated on me with and I’m still alone with no one. I was on a dating app and saw that him and his gf had a joined account on it because he viewed my profile. And I just can’t help but wonder if I just gave in and had three somes like he wanted, or allowed a third party into our relationship then maybe I would still have him with me. My kids would still have their dad inside the home. I can’t help but feel sad. I deserve to be happy but yet I’m not and the person who hurt me so badly is getting everything he ever wanted and it just hurts so much.