Oh honey. Believe me, everything WILL be ok.
I spent all of my second pregnancy sad, anxious, and panicky. I thought it was the biggest mistake of my life, having another baby. I felt painted into a corner, and very scared and alone. I was worried about my kids, my marriage, my sanity. A LOT of it was hormonal. Some of this played out further after my daughter was born, when I was diagnosed with PPD.
Fast forward to now - my daughter is almost 16 months old, practically the dictator of this house, and we all love her to pieces. My older boys didn't skip a beat when she was born - they just jumped right in and welcomed their little sister with open arms. To this day they run into her room every morning to say good morning to her. Always kissing and hugging her (annoys the crap out of her). They are FINE.
I do have days where I feel I have a lot on my plate (some of that of my own doing
), but it's a busy that I love and wouldn't trade for the world. Having three is the BEST thing that's happened to me. In a few months you'll be saying the same thing.