struggling to stay sane with newborn

i have a 3wk old and she is my whole world. i love her with every bone in my body and i’m so grateful i was given this gift. but i’m struggling mentally. my SO was able to stay home for her first two weeks and went back to work last week. thankfully my father had to stay home last week and was able to help out. everything had been going smoothly w a few rough patches here and there. today was my official day alone with my baby girl. hardest. day. ever. she cried all day. i would feed her, change her, burp her, and do everything i could think of to soothe her. i’ve been having trouble with my supply and have been having a bit of forceful let down making her frustrated when nursing. i just don’t know what to do. i have so much anxiety and i’m just so frustrated with myself bc i’m her mother,, i should know what’s wrong and how to fix it but i don’t. i feel so alone and my SO doesn’t make it any better. he thinks it’s easy and that i shouldn’t feel the way i do bc i’m basically asking for it when i have the option to pump or feed her formula. i don’t want to and he doesn’t understand that. no i don’t expect him to grow tits and feed her breast milk but at least a “you’re doing great” or something that won’t make me feel like i’m completely failing as both a mother and partner. i feel so weak.