Follow your instinct but remember the effort you put into finding a new job. Its not easy and people dont do it without a reason so what was yours?
Have you changed jobs before? Did you have this reaction then? How did that turn out? What impact is that having on your current thought process?
Before you spoke with your manager, how were you feeling? How much has she influenced your decision? Was what she said fair?
Has handing in your resignation made it "real" and a bit scary? That's normal. What was your immediate reaction on being offered the job?
Close your eyes and imagine going to work in 2 years time. Literally visualise your alarm going off, pressing snooze, feeding the cat, the smell of your deordorant... every single step. Dont over think it, just go with the flow all the way through to arriving at work. What workplace do you see first?
One thing I do that is actually really helps focus my mind on a difficult decision is flip a coin to decide.
Heads - I take the new job
Tails - I keep my current job
Your immediate reaction to whether its heads or tails eg relief or disappointment will tell you a lot about what you want without all the other noise.
In the meantime I would speak with your manager and tell her you are having a wobble and can you have a few days to really think it over. Dont tell your new employer until you know either way.
One place Ive worked at, once you've handed your resignation in, that's it but in most places there is some time to revoke it. Lots of people turn down new jobs but out of courtesy you need to tell them soon if you decide that.
Good luck whatever you decide.