I'm currently going through the exact same thing and I don't know what to do.

Me and my ex broke up 7 months ago. It was a sudden thing, he left me after 3 years. We didn't speak at all afterwards. I heard rumours he went round and told people I cheated on him which is why we broke up but that wasn't true. He left me because he said he didn't feel a spark anymore. I though he left me because he didn't want a girlfriend anymore. Anyway we didn't talk, I removed him on social media because he was being tagged in posts by a girl and it made me feel uncomfortable. My ex is a shy boy. He doesn't talk to girls, he is happy to sit behind a computer and play games all day by himself and I didn't mind that. I loved him for who he was.
7 months later I find out he has a new girlfriend and they have gone on holiday together - to the same place he took me on our first holiday. We were officially together in April and we went in October. Now how long have they been together for him to take her on holiday so soon. He got with her so soon after me. He told me he wouldn't be with anyone for a year because he was going to find it hard to get over me but here we are. I haven't been with anyone since. I can't do it. I cringe or feel sick. Anyway I thought I would do the mature thing and text him telling him I was happy for him even though I wasn't because I want to move on. He gave the shortest reply ever but it doesn't matter because he obviously didn't care that much about me to move on so fast.
It's been 4 days since I've found out. I haven't eaten. I keep throwing up. I'm nausea, I'm tired, stressed and it is because I can't stop thinking about how quickly he replaced me. I just feel like I didn't mean anything to him and he meant the world to me.
I want to move on. I want to stop feeling ill and get on with my life.

Please help.