Hope you are ok. Lots of mums feel the same before baby number 2. It’s hard, emotional, a big change for all (even harder to manage your own emotions when you’ve got another little one to rely on) and far more chaotic.
If you can, lower your expectations completely. I don’t know how old your first child is, but I had 2 under 2. Every day I wrote down something that went well, so that might have been everybody was fed, or I managed both of them crying at the same time. Chaos happens, it’s inevitable and it’s fine, we all have days like that. Bad days I took hour by hour, I soon got through them and if we had a bad patch where everyone (even me) was crying, no doubt an hour later we were all playing or snuggling together and the kids had both forgotten it. If most people get most of their needs met within a reasonable time, that’s great. Accept some needs will be unmet, or met less well (naps not being great, cereal for dinner for 3 nights on the bounce, someone crying for a while until you tend to the other one) that’s fine, you’re doing your best and everybody knows that, especially the children. That’s what they will grow up to see.
Don’t have any expectations regarding how the older child will take to the sibling and the new changes. Mine was fine with me but never acknowledged his sibling for 8 months, never tried to hurt him. Some people say there is a lot of aggression / hurting from the older sibling if they are a toddler, you can’t predict it. They didn’t choose the sibling so I just treat them as two separate individuals and forced nothing, my rule is you don’t have to like each other but you do have to either be kind or stay away from each other.
One to one time with the older one if you can, 10 mins a day is all it needs. I made a big thing of asking for help with easy things (choosing clothes) but remembering they are still my baby too and treating them like that,
never saying you’re a big boy now or anything. She is still your baby and always will be. But to make the
emotional aspect easier, the baby will love her more than anything and your daughter is getting a whole other person to love her like no one else ever will. (My little boy aged 2 says I don’t want you, I want *brother’s name, aged 4) sometimes when he’s crying!
Look after yourself, you are, in many ways, the most important person because you need to have most needs reasonably met to be able to recover from birth and thrive. Accept your emotions as part of the hormonal ride / natural huge change in your life. It’s normal, it’s something you will come out of the other side from.
I hope some of that is useful. You’re going to do great x