I just went back to w*rk after about 6 months of freedom. I was pretty apprehensive about going back, but I got into it and I'm enjoying the company (the company of my co-workers).
I think the biggest problem, was that I have spend so much time working toward FIRE that I never took much time for life and living. If you have the reputation for being a workaholic - watch out! You'll be free and totally bored and basically wasting away.
So, with this realization, I'm really making an effort to keep the 9-5 as only a 9-5 and skipping out on any side or freelance work that may come my way.
Going back to work however has given me an extra income boost that I will use to outsource my FIRE ambitions with side businesses which require little to no time to manage.
Here's the recap:
1 month off is great...
2 months awesome...
3-4 months started to wonder what the heck I was doing... was sleeping in very late every day - basically being a bum
5-6 months: realized I was turning into a bum, and started to get on track, joined a meditation group, started working out, cooking, reading etc.
Good luck to you. I suggest 'the Joy of Not working' as a light read for anyone preparing for FIRE whether it be a short term or long term engagement.
Cheers.