Should I negotiate my severance? - first time posting

I was offered 15 weeks of severance after 15 years as a teaching assistant, and I have a few days to sign. It has been a long 12 weeks. There is a non-disparagement clause. Did anyone negotiate and succeed?

Thanks for writing this out. I went through something close to it last year, and it got easier once I kept a simple log of every call and form.