Hi OP. Congratulations on your new arrival. I think what you're feeling is so very very normal and something a lot of people experience.
You're going through so much right now and feeling weepy, unsure and even a sense of 'what have I done?!' is most likely down to hormones, exhaustion and a bit of the baby blues at this stage.
I've posted similar before but I worried about this too when I had my DS2 as I'd considered sticking at one for a while. I'm so pleased I did though (he's now 8 months) as he and DS1 have such a great relationship already. They make each other laugh and I often catch them playing little games together and giggling away. DS1 is super protective of his baby brother and he has become DS2s favourite person who he saves his biggest smiles and laughs for. It's lovely to see.
I know there will probably be fall outs along the way and it won't always he smooth sailing but I know overall it's worth it.
Give it time for your emotions to settle and I'm sure you'll start to see your second as something wonderful you've been able to give to your DD, rather than having taken anything away.
People say love isn't finite and it's so true - you wont need to split the love you have for your 1st, you'll have a whole new set for your second and a unique relationship with both. There's a poem about this that I love and always makes me cry but I used to read it if I was ever unsure about my decision!
Admittedly time can be trickier when you have to juggle two, especially in the early days but you'll soon settle into a routine and get more time back with your DD to give her lots of cuddles and attention too. When you can, try and carve out some one on one time with her whilst baby naps or someone else takes them for a walk etc so she has your undivided attention, even just for a little while.
I'm sure your heart will soon melt when you see how they interact and any worries will be forgotten, give it a bit of time, keep talking and be kind to yourself.