Discretionary or systematic, pick one and stick to it. Systemic don't deviate from plan. Discretionary yes adjustments can be made as new information (price) forms. Find out which you are better at and stick to it. I prefer discretionary and "deviations" from plan, aka updating plan.
I have a target set at each trade. On rare occasion, if the price is moving aggressively to my target with a lot of momentum, I may move the target out a slight bit, and as soon as price crosses beyond my initial target level, I close out with a limit order. Usually it's an extra 2-3 seconds and 1-3 ticks. That's about as daring as I get. I keep track of these occasions and "extra ticks" and have determined that, for me, it's correct.
Exit half at your pre determined exit. Bring stop loss to break even on the rest. Exit the remaining half on a trailing stop AND/OR discretionary. I had a trailing stop on my remaining short YM position today, i was watching the market, something clicked in my head and i did an discretionary exit at 24100. The low was 24086. Market went up 300 points in 4 mins after that.
I always have a target within my plan, in other words: I always get out when my target is reached. However I have rules too to get me in back again if it moves strongly but then again I have a new target.
Consider refining your trading plan by defining & applying specific management strategies to address the very scenarios you're referring to. For example, when it gets to (or within X% of) your pre-defined target and is exhibiting that type of behavior: switch to a tighter trail using a pre-defined parameter (e.g., bar-by-bar, MA, etc.), possibly in combination with a faster timeframe; “lock in” partial/full profit (literally or via hard stop), and then apply pre-defined management to your remaining open position; or some combination of the two. Management beyond your pre-defined target could even be semi-discretionary -- if say for example, you’re skilled at reading price action. Backtest the proposed management, and then implement it if the stats support doing so and it's compatible with your personality. Essentially, you'd be preserving the spirit (and profits) of your original plan/proven edge, but tweaking and improving it by giving winners a chance to run beyond your original target.
When you first start have the openness to change your system So it can change and grow as you gain more experience and knowledge. I think you know a system is important but yor having trouble cause your system doesn't match you opinion during times you indicated in your post. When you are doing something different that your system, its signaling that something needs to Be optimized... either the timing part of your system, the way you deal with your emotions part of your system or the money mangement part of your system. It's part of the process of creating a system and being a trader in my opinion.