I’m a terrible mother

I have a 4 year old and a 5 month old. I am realizing I treat them both different and I don’t know what to do. Basically, I am always telling my son to go to his room and stuff alone because he wants to be in my room with the baby while I’m trying to calm the baby/put her to sleep/not overwhelm her and he is very loud and I can’t have him around her when I’m trying to do those things unless he is going to act calmly and quiet around her. My husband mentioned to me today I need to treat him better because he feels like a third wheel basically and I couldn’t believe it, is this really how my 4 year old feels? Because of me? I’m terrified he feels rejected because I give all my time to my 5 month old while telling him to go off and play by himself and I’m so stressed and tired I just feel like I don’t have the ability to focus on both children at once, I’m a terrible mother now. I really need advice because I don’t want my son to feel the way he’s feeling because of my inability to parent correctly. What do I do?