I am at a loss of what to do. I had started working on this project, a systematic review and meta-analysis back in undergrad. I was a senior. Long story short, for a while my manuscript was going well. Come August 2016, my mentor stopped communicating with me in the final stages. Weeks of no responses predominated, and as a result I emailed the Chair of the department on 3 separate occasions (over a total of 7 months). I was promised 3 different deadlines, all of which went unfulfilled. I finally emailed the dean in April when we got close to my final deadline of May 1, 2017. I got my edits, but I was 3 days away from leaving the area because I was heading off to graduate school.
I ended up telling my mentor I didn't want to publish or take this situation into a new institution, but if I found someone who could advise and help me with the situation I would reach out to her. I expressed I had felt we had failed to meet the deadline and the outlook was bleak. She responded positively saying we hadn't missed the deadline, but if I were open to forfeiting my primary authorship stance then she'd be glad to completely finish the manuscript. I took this as a direct offense to the work I had done because I had decided on the topic, done the data extraction, results, conclusions, and limitations of the paper.
I have found someone but am worried my old mentor will not follow through or will hinder the process again. My new adviser suggested I send her an ultimatum and deadline for her final comments for the manuscript and submit to a journal. However, as I am not a researcher, am going to grad school to be a clinician (current graduation 2019), and only have a bachelor's degree, I am scared if I do this then I may lose her support for the manuscript.
How should I proceed?