Scaling up/down positions post-entry without blowing up?

Hey everyone, fairly new here, been demoing for about 6 months now on $EURUSD and some commodities. My biggest hang-up right now is managing positions after the initial entry. I'm okay with the initial risk sizing based on my stop and account size, but when a trade moves in my favor, I struggle with whether to add to it or scale out. My gut tells me scaling out is safer to lock in profits, but I often kick myself when the trade keeps running. Conversely, trying to scale in when it's going my way often leads to me adding too late and then the trade reverses, wiping out initial gains. How do you guys decide when and how to adjust position size after the entry? Is there a systematic approach or is it just experience and 'feel' that tells you when to press or pull back?