Struggling with how fast my ex moved on after our relationship?

So to cut a loooonnngg ass story short, I finally gave myself emotionally to a female after being hurt 5 years prior by a girl who "cheated on me with her ex" long story short, and i never wanted to make myself vulnerable again like that, reason being why it took 5 years to open myself up again. I put everything I had into this relationship, Got a great job, bought a new car to see her more. Redecorated my whole apartment to how she liked it. Bought her "Everything she wanted" and only dined at the most expensive restaurants. I got a little upset over time because she was communicating with her ex quite frequently and it hurt me, so I got a little insecure well maybe more than a little, After all he was asking her if they could be "fuck buddies" but she continued chatting even though I feel that isn't something that should be acceptable in a relationship. basically after everything it got to much and our relationship ended after a year. she was really upset and she promised it would take her a long time to move on from this and that she wouldn't even be thinking about seeing anyone else, anyways I found out she was already dating someone a month after we separated. Like our relationship was fuck all. she didn't give me anytime to breath and it completely suffocated me the thought of her being with someone else. It hurt so god damn much. its like the worst feeling i have ever had. I haven't even considered dating anyone. I was damn near dead inside as I felt like my whole life had ended.

My question is how can someone move on so quick? her track record worried me in the first place as she has been with 4 different guys in 4 years now onto a 5th? how the fuck can people live like that? it baffles me to no end. She was my whole life and she just moved on!
Struggling with how fast my ex moved on after our relationship?
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