Cheating BF in 3 years of dating

My (29F) boyfriend (34M) has been cheating on me with his Baby Momma who lives 600km away. I got her numbers and asked her and she said they've been together for 10 years & have never broken up. I've been with him for 3 years and we've been living together for 2 years. I called her in front of him and he refused to talk, but muttered that he wants me. He just kept pacing around drinking water. I packed my items and went to my place. How do people get over break ups? Also why would he do this? I'm shattered.