How do you conquer the fear of gaining?

I'm not sure your feelings are irrational. You were once fat without intending to be so it's a fact that it's something that can happen. It's not a fantasy or really very far-fetched at all; it is actually a very common thing for dieters. I'd say this makes it a perfectly rational concern.

This doesn't mean it has to disrupt your life though. I worry about getting fluffy again but it doesn't stop me from doing much unless you count choosing to recomp rather than bulk and cut as "much". I deal with it the same way I dealt with it while losing: by taking it day by day and meticulously observing the good habits I have formed more often than not.

You seem to have all the required knowledge, you just need to get adjusted to and gain confidence in the new way of things. You might find, as I have, that maintenance isn't much different than losing in the grand scheme because, when boiled right down, it's simply a constant process of losing and regaining the same last 5lbs over and over to maintain your preferred weight range.

I think that's pretty normal early on. As time passed, that fear became less and less as I realized that so long as I was maintaining the healthy habits I had learned while losing weight that maintaining my weight was pretty easy. 5.5 years later, I don't really think about it much anymore. I just eat well for the most part and stay active and the rest tends to take care of itself.

Over the years I have gone up and down a bit...in winter I usually put on about 8-10 Lbs because my activity dips, but I don't let it go any more than that. This winter I'm up about 5 Lbs from my maintenance and I'm aiming to hold it at that this year.

As long as you're monitoring the scale, you have full control of your cutoff point...like I said, for me that is 10 Lbs...that's the point where most of my clothes start becoming uncomfortable.

Gaining 15# in one year got me over the fear of gaining. I failed. I accepted it. Now I just want to manage a weight that allows me to wear my jeans and good winter coat without busting the zippers.

Congrats Gary - huge achievement.

I like the Prof Steve Peters (author of The Chimp Paradox etc...) mind model of chimp, human and computer.

At first every fluctuation elicits a chimp response of irrational fear, recognise it as such. Use logic (your human brain) to soothe and reassure the chimp. "Yes chimp I'm up a couple of pounds but we haven't had a 7,000 cal surplus have we?"- trust the maths and the process.
(Do I remember correctly you are a Science teacher? If so, your human brain has some powerful soothing logic.)

The power of repetition and habit isn't to be underestimated. A bit like the first day in a new job or first time you drive a car etc - terrifying! Yet over time it becomes normal. Same really for maintenance - what was once new is now the new normal. It is a skill to be learned and will probably evolve. I've loosened my controls but still periodically review if I've become complacent. If you drift over your range take action - it doesn't need to be drastic and should be considered action. Think about why a drift happened and if it's acceptable or not. Personally there's a seasonal or vacation drift I will accept along with the need to resolve it afterwards,

Vigilance is of course required but not white knuckling emotional fear, maintenance should be a happy state.

I am with you Gary. I am within a few pounds of my goal weight. I lost over 200 lbs. and gained a significant portion back and have lost most of that. I feel strong and fit (except the recent rotator cuff surgery), but I also share your fear of gaining again. I understand the process; I am nervous nevertheless.

Don't get caught up in the minutia. Your weight will fluctuate hourly, daily and weekly. Just like when you were losing weight it's the trend over time that matters. I set a maintenance weight zone for myself. My ideal weight, and then 5 lbs above that. As long as I stay in that range, I'm good. If things are trending outside the range for longer than a few days, I know it's time to tighten up my logging again.

Rinse and repeat.

Gary, amazing job and welcome to maintenance. I’m just wrapping up year two and will tell you I white knuckled the entire first year. Year 2 has been a wee tad easier. My advice..... keep doing exactly what you did to lose the weight with a handful more calories. Keep your foot on the gas until you feel 100% solid (this coming from someone who gained back 75# the first time).