We were, by all calculations (close to $2m across the accounts, paid off house), more than OK to FIRE, but pulling the trigger was hard and a total leap of faith. It stands to reason. We all got here by basically hoarding our money and being frugal. We have savings because we didn't touch them unless absolutely necessary. That mindset does not change overnight. "Using" your savings in any capacity is a scary, unfamiliar and -- to our way of thinking -- historically dangerous concept.
Two things helped me (I was the second to go, one month after DH became FIRE) take the leap:
1. A friend in a much more precarious situation (house not paid off, with MS and higher health costs) and her DH had left stable, good paying jobs for self-employment and more free time. Due to her illness, she has no idea how much more time she has left at a decent quality of life (nor do any of us, but it's front and center for her). She made me pick a date and commit that quit date to her, after we'd had an hour long Skype chat about what was holding me back. Without her, I probably would have lingered on at my horrible, high paying job with great benefits until some other external, precipitating event made a decision for me.
Do you know someone who might be able to do the same for you?
2. Estate planning. As we do not (and will not) have kids, it was time for us to make arrangements -- advance directives, wills, a living trust. Otherwise, our poor family members couldn't even sell our house, would have to deal with probate for years, ALL of that. We had to decide who would get what, and that's what did it for me. We'd saved all of this, only to give all of it to other people? (Which is fine, but...) We couldn't think of any "stuff" we wanted to spend it on, like most Mustachians. We do not want a Tesla, a second home, etc. The ONLY thing we wanted was more time. If we didn't start taking it now, then when would we? For whatever reason, just seeing the estate plan was a forcing issue in getting us to use some of our savings for ourselves. We bought our freedom, health and time instead of stuff.
We have not regretted going FIRE when we did for a single moment. I have had a few small panic attacks, but those are all over loss of identity, not being a "productive member of society" for the first time in my life, etc. It's not regret, it's just getting used to a new identity and figuring out what that is... and THAT is SO much scarier than the financial calculations! :)