As long as I've owned my car, (4 years) 1st gear always grinds. It does not grind when I try to place it in gear from a complete stop. It will only grind and not go into gear while *rolling*. No other gears have problems. What could be the problem?

I've downshifted into first while rolling since I bought the car and I've put over 100k on it personally. First shifts fine. I've even dropped it into first on tight corners and hairpins on the racetrack, no grinds. Shifting into first while the car is moving isn't the issue, usually it's ramming the gears and beating the synchros, or shifting from a rolling reverse into first (intentionally broke a tranny doing that)