I was in your exact same shoes. The first year of my dd2 life was pretty sad for dd1. I didn't spend as much time, have as much patient, I felt very sad for her. My dh and I tried to make sure she got some one on one time with us, but she still had some regression. We dealt with a lot of anger and our schedules pretty much went to crap. That first year..ok more like 9 months were pretty crazy and miserable.
Good news is, it gets better. No, really. It does! As dd2 got older and more indepedent(as in more mobile and able to go longer between feedings) dd1 was able to connect with her sister more. She loved her baby sister to start with, but being able to interact with her and play with her made a huge difference. At 17 months, they get along great.
We still have issues with dd1, but they're more age 5 issues than attachment issues. Its only been in the last 6 months that I have been able to feel positive towards my dd1. I know, that sounds horrible. I loved her, but I didn't like her most of time. I'm so thankful that we have reattached.
As for the nursing, my dd1 was nursing up until a month after her 4th birthday. We had a situation with her mouth that prompted cold-turkey weaning. I'm sure we could have continued but I couldn't. I just couldn't nurse two kids anymore. If you can keep going
if you can't, 3.5 years is an amazing run.
Just remember that some of the issues you are facing isn't just that there is a new baby in the house. She's turning 4 and thats a whole new ballgame.
If you can, try to keep a loose schedule to your day. Try to have some special one on one time every day. Family rituals can help, like tea time together. Or reading a story together while baby is napping (if you're like me, babe in arms
) Try to get your daughter involved by asking her help for small things. Accept her answer if she says no.
Ditto to pp, you will get through this. You will get frustrated, and feel like a failure, or your dd will adjust asap and become the dream big sister
Only time will tell, and you'll still get through it.
Congratulations on your new baby!