One thing i have to say, is don't regret anything as it all happens for a reason! To be honest, you don't sound like a whore you just wanted to be with another guy and it's only understandable. Personally i don't think you owe your ex anything, as you have no need to talk to him about your sex life as it is your business, which you can keep to yourself.
I know you say you feel bad but that is because you know that your ex boyfriend still likes you, however there isn't anything that can be done about it's something which he has to deal with. One thing is that at least he was honest tat he knew not many guys would be.
Don't feel terrible you are single and have none to answer to. So try not to regret what happened as we can't change the past however we can change our future, but that's if we want to.
Hope it helps ever want to talk I'm here.