Scaling out on DAX profits – am I overthinking it?

Been trading $DAX for about six months now, and I'm finding entry points I'm reasonably confident in, but my exits are still a mess. Specifically, I'm trying to implement scaling out on partial profits as the move progresses, but I often end up leaving too much on the table, or worse, seeing the remaining position stop out for a net loss on what was initially a winning trade. I log everything, but it's hard to discern if it's poor execution, flawed initial targets, or just market noise messing with the strategy. For those who consistently scale out on European indices, what's your primary determinant for taking those partials off the table?