Horrible mom

Ladies I need help. I feel like a horrible mother. I have a 7 year old and a 2 year old. My son is the oldest, and I just don't have a connection with him anymore. It makes me feel like a horrible mother. He has ADD and is on medication, and I feel like I'm constantly yelling at him because he doesn't listen. It doesn't matter what I do, he just doesn't. Here lately because I am pregnant I can't stand him not listening. All I do is yell, and can't wait for him to go to his dad's. I don't get upset or feel the same about my 2 year old. What is wrong with me? Has anyone else gone through this? I don't want to feel like I don't like being around my own son. And I would appreciate negative comments to be kept to yourself. I'm trying to better myself and fix the problem.