your granpa is right, but here are some advices:
1) write when you are up or down (emotionally).
2) once you start, new ideas for new songs will come into your mind just write them down for the future , they'll serve you.
3) forget about the time
4) write all the lyrics that come to your mind once you start, you'll always have enough time to delete what you don't like.
5) try to forget the songs you like and do what you are good at (don't try to play eric clapton if you have a beginner's level), and never compare your songs to those of your idols.
6) don't complicate your life with difficult music, strong lyrics... etc, but put your feelings, that's the most important thing.
do what you know , love what you do , and use your feelings and you'll do a great job