One thing I did that was surprisingly helpful was to change my direct deposit and transfers around.
Most of my working life, I had my paycheck deposited into my checking account, and then moved money from there to savings and investments. Somewhere in the last few years of working, I changed it so that my paycheck was direct deposited into my savings, and then whatever I needed for spending I transferred from my savings to my checking.
When I went to actually retire, it was a much smaller step, because my paycheck went away, and I just refilled my savings from my investments. I was already refilling checking from savings, so that was no different.
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A second thing that helps is to actually sit down and determine exactly how your FIRE plan is going to work in detail, rather than just handwave that you spend less than 4% of your stash. Which accounts will you pull from? How much? How often? What method? This makes the whole thing real and gets your brain more used to the idea I think.
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Finally, as someone mentioned above, time to see it actually working. I retired in 2016 and have been invested mostly in US stocks, so I've been fortunate that my investments so far have gone up faster than I have withdrawn and spent. So that helps me think that it is working OK.
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Finally I'd say that even though most days I feel confident, and mentally I know that I have more than enough, there are those times when the IBL (Inner Bag Lady) feeling shows up. When this happens I think I probably need to look at where that's coming from and if there's anything I can do about it. The IBL feeling doesn't happen very often for me, so this hasn't been much of a priority.