Here are some quotes that really inspire the mentality for self-development:
"Far better it is to dare mighty things, to win glorious triumphs, even though checkered by failure, than to take rank with those poor spirits who neither enjoy much nor suffer much, because they live in the gray twilight that knows not victory nor defeat." --Theodore Roosevelt
"Some day, in years to come, you will be wrestling with the great temptation, or trembling under the great sorrow of your life. But the real struggle is here, now, in these quiet weeks. Now it is being decided whether, in the day of your supreme sorrow or temptation, you shall miserably fail or gloriously conquer. Character cannot be made except by a steady, long continued process." --Phillips Brooks
Feel free to add some more inspiration onto this thread