Separating after 2 1/2 years and recent engagement

My now ex and I were together 2 1/2 years, recently engaged. Now she wants to give up and leave. It’s sad cause we have a child together, I’ve been a father figure to her little sister she has custody of, and she’s been a mother figure to my other two kids(who’s mother hasn’t been in their lives). It’s a whole mess. We got engaged 3 months ago. Less than two months later she wanted a break, soon after she wants to move out, a couple weeks after that she’s talking to another guy(while we’re still living together), and now they are getting serious. I’ve never hurt so much or felt so alone in my life. I’m starving for affection right now and she’s gone cold. I haven’t been the best partner so I can take partial blame for this. But I’ve always been loyal to her. I don’t deserve this. Everything fell apart so fast. She’s gonna leave soon, and my household persons, and material things will cut in half. It will feel so empty. I’ll be alone and also thinking about her with him. I can’t eat, can’t sleep. Nothing brings me joy. I’m falling apart and am at an all time low. Trying to stay strong for my kids. I don’t really have any friends or family around here anymore. This was my family and she was my best friend. I’m hurting so much. The days are long and the nights are hard. I was so happy only a few short months ago. So much has changed so fast. I’ve been thru this similar situation three times in my adult life now. Everyone gives up on me.