Tired of feeling like a shit mom

Idk what going on with me I’m so down all the time but I’m happy if that makes sense like I don’t really show how bad I’m feeling because in the back of my mind I know there’s no reason to feel like that. And now I feel like a bad mom like my son is sooo perfect he deserves a better mom than me I get so frustrated with him sometimes and get angry then make myself stop to think like how can I get so mad at him he’s just a baby and looks up at me smiling while I’m over here pissed off. Idk if it’s something deeper and idk if postpartum depression can come on 6 months after having a baby. I don’t feel like I’m good enough for him for anyone at that. I lay in bed every night and just cry and I can’t make it stop and I don’t know why I’m even crying. I love him more than life itself and still don’t feel like I’m enough or good at what I’m doing or know if I’m doing things right. I feel so alone