How can I get my ex husband back?

Me and my ex husband got divorced 6 months ago and I'm desperate to get him back. I love him and miss him so much and its killing me that we're apart after 7 years and 3 kids. Like 2 months after we got divorced he started dating his girlfriend and I absolutely hate her. We were separated for a few months and we had plans to get back together but then she convinced him to get a divorce the little home wrecking tramp. He said they were friends before we got divorced and she was looking out for him since I was such a terrible person but I don't buy it. I admit I made my mistakes but when I said my vows I meant for them to last forever. It was my fault we were separated but I thought he was over it. About 5 years ago, he got a new job that required him to take frequent trips out of town, and every time he came back he would just argue with me so our happy marriage was coming to a crumble. After a while, I did the unthinkable........I slept with his brother. It was a mistake, but it kept happening and I couldn't help myself. I thought I was in love with my brother-in-law but I stilled loved my husband so I stayed with him. When I got pregnant with my 4 year old son, I didn't know who the father was but I couldn't tell my husband that so I didn't. After my son was born me and my ex were still having problems but I loved him so I stayed but I kept having a ual relationship with his brother for 4 years. One day at a family function his brother got completely wasted and told my ex and my ex started calling me a and a b*tch and it was so embarring. He wanted a DNA test on our three kids and at first I talked him out of it, but his family members told him to do it. Our 4 year son and 2 year son were his but our 1 daughter is his brother's which is ironic because I was sure my daughter was the only one that was my ex's. It was humiliating and I thought we could work through it but he just wasn't trying to. Him and his brother don't talked anymore and they used to be close but its not like its my fault, his brother took advantage of me. His brother and I are not together anymore so we can't we let the past be the past? He's a great father to our kids, even my daughter, but its not enough because I want him back. How can I get him back?