The fire and the rose - David R. George

Spock, displaced in time, watches his closest friend heed his advice by allowing the love of his life to die in a traffic accident, thereby preserving Earth's history. Returning to the present, Spock confronts other such crises, and chooses instead to willfully alter the past. Challenged by the thorny demands of his logic, he will have to find a way to face his conflicting decisions.
Once, he preserved the timeline at the cost of Jim Kirk's happiness. Now he is forced to re-examine the fundamental choices he has made for his own life. Unwilling to accept his feelings of loss and regret, he seeks that which has previously eluded him: complete mastery of his emotions. But while that quest will move him beyond his turmoil, another loss will bring him full circle to once more face the fire he had never embraced.