Not how long, but how well you have lived is the main thing.
To get the full value of joy, you must have someone to divide it with.
Life is like riding a bicycle. To keep your balance, you must keep moving.
This one is really important, especially in times when you are not that happy.
Do not make the mistake and curl up at home, thinking about your life.
STOP THAT RIGHT NOW!
Go out, start hobbies, do sports, work, whatever. Keep going, keep yourself busy and you will see that happiness will find its way back to you.