Gaining

This is just my thought, but here's my best guess.

You keep coming back to it, right?

This tells me you don't hate the thought of gaining, but that you love it. What you seem to hate is that you do love it. But perhaps you still feel shame. That on some level, you aren't supposed to love it, but hate it. Then those feelings come bubbling up to the surface after the feeling of being turned on is gone.

This tells me that while it may take a long time, the easiest thing is to accept it. And the sooner you accept it, the better and the happier you'll be.

I often regret that it took me far too long to accept this. What if I accepted this earlier?

Another thing that helps is to realize the average American man aged 20-59 weighs around 202 lbs last I checked. And 250 isn't all that uncommon. 300+ may be less common but it's still common enough no one thinks twice. You'd have to gain a lot before you really stick out in public.

Hopefully something I said is helpful.

The thought of the gain arouses you more than the reality. It is precisely "post nut regret" (great term BTW) which can happen when a source of stimulus becomes far less desirable post climax. For me it's usually women I can't and/or really don't want to have. For you it happens to be the weight gained.

How long have you been gaining and how long have you been aware of your desires?

Your "post nut regret" may be part of this that you haven't accepted yet. When I was first exploring this, I had the same sensation. Afterwards, I wasn't at all attracted to the women I was looking at and felt ashamed that they were what got me off. Part of me hadn't really accepted this yet. It actually took me quite a while to really lean into this, but once I did, the opposite became true. In fact, I wanted more. Even right after, when the absolute peak of interest and raw urge was gone, I still was interested.
Now, your situation is a bit different as you're the one gaining and that comes with its own hang ups. If you focus on why its a turn on and fold that into your identity rather than feel ashamed, I think you'll feel differently. Its not exactly like a switch flip though and us easier said than done, but you'll know the difference.
Obviously this is part of you. You keep coming back to it even with the knowledge that afterward you'll feel guilty. One could argue that to be a sign if addiction, but it sounds like you've made more conscious decisions than mere impulse. So many people have this fetish or something like it or even something completely different that they feel too ashamed to ever act on. You made the decision to be happy and fulfilled. You're living the fantasy. No need to feel shame or regret.
If you don't truly like the life style, you can always adopt another and still feel good about the fact that you tried it. You lived out your fantasy and had your fill of it. There's no shame in that either. Just enjoy whatever you do and let it be part of you.
If I'm completely off base, I'm sorry. I can only speak from my experience and not fully accepting that I loved fat women was what caused that sensation for me.

As much as I'd like to give a clear easy answer, I think its more of a personal thing. At the risk of going full drama llama, I'll say its sort of spiritual. Only you can know how to accept and incorporate a part of yourself.

Talking about it with other people helps. The more people see this as part of you, the easier it is to see it as part of yourself.

Might be just as simple as that life style isn't for you then. If that's the only reason you gained any and just that little bit of change makes you unhappy when not immediately engaged sexually, then that may be where the line between fantasy and reality is for you. Gaining weight is a fairly impactful lifestyle shift. If you want the freedom to eat what you want, like the constant changes, or any number of other reasons that persist throughout the day that justify it for you, that's one thing, but if all you get out of it is occasional sexual pleasure followed by immediate regret with no other redeeming qualities, then it's not worth it.
I know for me, I don't like being overweight. It happened naturally for me over the years, but I'm taking steps to lose weight currently. That said, I do indulge in the fantasy of just letting go from time to time, but I know the impact it would have on my life and for me the cons far outweigh the pros(if you'll excuse the pun). It seems you might be in a similar situation.

Personally, I find I’m more likely to experience post nut regret if I’m single or with a partner that isn’t into bigger.

If I find I’m hitting post nut regret, it’s time to give gaining a break—not necessarily *lose*, but just stop gaining for now. Just hit this phase recently after a gaining phase.

Post nut regret is fairly common :/ the brain does a LOT during sexual activity. For some people, post nut regret is just a form of exhaustion.

Pay attention to how long it takes to pass. If you get off in the morning and want to lose the rest of the day, then maybe it’s time to stop gaining.

But if you find the feeling passes fast enough, and you like your size even when you’re not sexually aroused, then maybe post nut regret is exhaustion.