Why is he putting me through this hurt?

I've been with a guy for 6 years. We have 2 children together. He and I have had a wonderful relationship up til the last yr. We have been arguing, and even went as far as seeing other people a few times. We were always miserable when the other one would decide to leave the relationship and see someone else. He knows how hard I take it when we break up, I sulk and get the can't eat can't sleep type stuff.

3 days ago he went out and done some pretty childish things, I told him that I needed him to stop the crap or I was going to have to leave him because it wasn't setting a good example for our children. He was mad but said he understands. The next day I didn't feel good so I slept for most of the day and when I woke up I had a text saying "so u r ignoring me now? Answer or I'll take it that we're over." I didn't hear my phone ring and 7 hours went by before I noticed the message. I immediately contacted him. He wouldn't answer me.

The next day he messaged me and said he had been with someone else and spent the night with her and I needed to leave him alone. He went as far as sending me very hurtful screenshots of their conversation. He called me 3 times yesterday and kept asking me how I felt about him and the new girl, then he would say "well I dont like her anyway so I ignored her all day, I am just going to be single" then he would switch it up and say he was just messing with me. I acted positive. It only made him furious towards me. He then said well so u know she is already falling for me in just 2 days and I'm getting feelings too. She is so perfect for me and has all the qualities I've always wanted in a woman. I remained positive even though I was dying inside. He spent another night together last night, he made sure I knew about it. He even went as far as telling me they sit together and talk about me, that I am 90% of their conversation because she is helping him move on from me. I can't stand this! Advice please! I love this man and I don't want this!
Why is he putting me through this hurt?
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