Is there something wrong with me?

I know this makes me seem stupid, but please don't judge, I'm just looking for a little advice. My boyfriend and I have been together for 4 years, and just had a baby together. he was the PERFECT man for me, and the only man I have ever loved, but for the last year hes been constantly flirting with other women, sending nude pictures of himself to other women on an app called snapchat, and even slept with my friend. I dont know why, but he just just continuously does it, it used to hurt really bad, I would cry, alot. Now it's like I dont even care, I dont feel anything, not sad, mad, just dosent even phase me. I'm so scared to leave him, we've been together for 4 years, but is it normal to just feel like that?