Feel like a failure

Hey ladies. Mama of 2 here, 3 year old boy and 5 week old girl. I am struggling to decipher normal 2nd baby struggles and PPD. My 3 yo is proving to be difficult, as we knew he would, and I struggle to keep my cool with him. Some days it feels like I'm always telling him no or to stop doing something and he just whines all day. I spend so much time frustrated. Then, on the other side, I feel so guilty for not giving my daughter the same attention I was able to give my son when I brought him home. This is normal, right? Crying sometimes is normal, right? I scored ok on my PPD screening at 4 weeks, but I don't want to ignore it if it's there.