On scaling out positions vs. letting winners run - a mental block?

Alright, so I've been wrestling with something that I'm sure many of you seasoned folks have already figured out, but it's really messing with my psychology lately. I've read all the advice: 'let your winners run,' 'cut your losers short,' the whole nine yards. And conceptually, I get it. But in practice, I find myself in a bizarre loop.

Say I've got a decent position going, $SPX futures, for instance, and it's up a good chunk. My initial target is still a bit further away, but then I start seeing a dip, nothing major, just typical market noise. And that little voice in my head, the one that probably went to business school, screams, "Take some off the table! Secure profits! Don't let a winner turn into a loser!" So I scale out, usually at some arbitrary level, and more often than not, the market then proceeds to hit my original target, or even blow past it. I'm left with a fraction of the profit I could have had, feeling a bit like a chump who snatched pennies while dollars were on offer.

On the flip side, when a position goes against me, I'm a saint with my stop-loss. No hesitation there. So why is it so much harder to let winners breathe? Is this just a common mental hurdle related to loss aversion, but in reverse? Or am I missing some fundamental understanding of how you all manage to scale out without sacrificing the potential for bigger gains?