Should I negotiate my severance? - 8 months and counting

I was offered 15 weeks of severance after 15 years as a medical assistant, and I have a few days to sign. I have been trying to stay organised about it. I do not want to leave money on the table. Did anyone negotiate and succeed?

I asked for two more weeks and a neutral reference. They agreed to the reference and added a week.

An hour with an employment lawyer cost me a small fee and made me feel much safer about signing.