Negotiating an offer, is it too late? - 10 months and counting

I want to negotiate but I am afraid they will withdraw. I have read a lot of threads here and could not find my exact situation. The benefits are decent. Did it backfire?

Keep everything in writing and save the confirmation numbers. It saved me later.

Thanks for writing this out. I went through something close to it 10 months ago, and it settled down once I kept a simple log of every call and form.