A couple of quick comments before I start 'work' for the day:
Are your boys grown now and did you factor in any financial help for them (University/House deposits etc)?
Yes and yes.
So far, I've got one son through uni and out the other side. He's now working, but had almost a year of unemployment/illness (he's better now) that I helped him through -- at one point, covering his costs was more than my living expenses! You have to factor in contingency funds. I've done some extra work to refill that pot. Good thing too as he's getting married next year!
My other son is at uni after a previous abortive attempt, followed by taking a year out to work and save. He has chosen to be at home while on his first year of a new degree. I don't blame him one bit, and I'm actually glad for the company. I help by having him here rent free (I know that mumsnet is divided on this, but halls fees are over 5k!), and I'll also help with his course's year abroad, if needed, but he's pretty much saved every penny, so I doubt he'll need much. He has a Gamestop share, so he's pretty savvy. 
Both of my kids are grafters. It's lovely to be able to help them, and I have the reserves to do so, but neither of them would ask for or expect massive house deposits from their single mum typing in her garret.
I think people tie themselves in knots about this when a little bit of struggle and a lot of creative thinking is very good for character development.
We just can't afford to do both - support the DC financially (until they're able to do so themselves) and retire at 55
Thankfully, FIRE doesn't have to be all or nothing. The good thing about school fees/uni etc is that it all comes to an end, and children eventually get jobs. You'll then have years to throw yourself at FI. In the meantime, it sounds like you are thinking seriously. Reconsidering your costs, and increasing your income short-term is never a bad shout. So is approaching early retirement with the anticipation of working one day a week, or so. Creative thinking helps here too.
One of the fallacies about FIRE that I read over and over is about a fear of reduced standard of living. All I can promise you is that I feel rich because I choose how I live now.
Back later to read!