The seedling crowded by success needs room as much as the struggling one.

Thinning is harder when everything is doing well. A tray where every seed took looks like a good outcome and is actually a problem, because the plants are now competing for the same light. The instinct to intervene only where things are failing means the crowding goes unaddressed until the whole tray is weak. Too much of a good result becomes its own constraint.

Look at where you have said yes to several worthwhile things at once. Removing one is not a judgment on its quality, it is what gives the rest enough room to become anything.