What toxic advice. Refusal to discuss us infantile and damaging.
OP, your gf clearly has insecurities (justifiably), and in most cases, we all feel some jealousy and insecurities when an ex is even mentioned one too many times, nevermind if our other half worked in the same building as his/hers. As adults we need to own our emotions, not deny them (before anyone comes at me about being totally indifferent to a lurking ex in any capacity). 3 days or 30 years, and ex is an ex.
If you think the world of her, talk to her. It may take a few conversations before you get on the same page, but you think the world of her, and I assume likewise, so don't go throwing it away like people are suggesting on her. You'll find on mn everyone's all about shotgun divorce and ending relationships when there's still some efforts and patience that can be exercised to ultimately stay with the person you love so dearly.
You absolutely should not be quitting your job. But certainly put in the work to get your gf on the same page, and draw a very clear healthy boundary of ex not being mentioned, and if there was any incident ever of ex crossing a boundary (even showing a remote interest in revelling in the past), you'd shut that down and tell your gf, to keep that trust. Please don't go saying you'd shut it down with ex, and gf simply needs to trust you. That's not how women work most of them time, and reassurances are always welcome and give a sense of security and priority.
If your gf is adamant without reasonable explanation that you should quit your job, then unfortunately you'd let her go over a job. Adulting is not child's play. Money makes the world go round, and in 2023, quitting jobs without a 'real' issue, especially in the name of love, is just juvenile.
It's not easy to find someone you think the world of. So hold onto that and don't fall so easily for the comments that are suggesting otherwise, without putting the world in, considering the 'issue' is from your history. I would suggest the same if the issue was from the gfs side.
Best wishes.