Most Helpful Guy
So you believe that you're cheating on your ex-boyfriend because you're going out with other people and while you're with them, you can't stop thinking about your ex? And, he's moved on but you haven't? At least not emotionally.
Well, to answer the question in your title, you feel like you're cheating on him because you still have strong feelings for him so it feels wrong to be with anyone else, even just from a dating context.
Your last question about why you haven't moved on from him can have many different answers, especially depending on how long it's been since you've broken up, who broke up with who, why you broke up, etc.
Most Helpful Girl
Its totally normal to feel that way, you get used to being w/your ex and its weird to try to connect to another person in that way. Understand that you are not cheating on your ex, you need to move on. If you keep feeling that way, you are not ready to date just yet, you can end up hurting another person and yourself.