You have a feeling of replacing him with someone else and the thought that he was recently the one next to you.Give yourself a little time and that feeling will go away eventually.:) It's similar to getting used to the light after the dark.
Well thats simple, even though the relationship was long over, your heart feels like you're still tied up to him. Find some hobbies, talk to old friends/make new friends. You can't be happy with someone else until you're happy with yourself It sounds like you still feel connected to him for whatever reason. Maybe some soul searching and some quality time with your thoughts will give you the answers you need. After breaking up with someone you can still feel like you are with them because it might be what you want to do even if they don't want to or you know that it would be wrong.
And no matter how much time passes, you continuously keep on thinking about him and about what you had with him as a couple. He may have been controlling in a sense as well, causing you to feel guilty for moving on. When you find someone else but you are still dealing with a breakup this can lead to what you are feeling.
Which is why, then when you start seeing other people, you feel as though you're cheating on him and the new relationship with the other guy would feel wrong. You have that feeling because he was recently part of your life, someone that was the one for you and even after a month you have the same feeling because it can be unusual change for you.
The truth is, this feeling of urgency and intensity or strong attraction toward another person is not necessarily a reliable indicator of whether you are in love or should immediately dive into a serious dating relationship.
The wisest man in the bible, King Solomon said, Guard your affections, for out of them come the issues of life.