My mind stop at 14 or 15...?

Past has been lived and holds no surprises or pain or fears. If the present is not so inviting one slips in past and feels happy ( like old folks). One tends to edit out the painful detail and feel nostalgic about the happy bits. Why should a 20 year old not do it? Cause you are in the beginning your life and should be out there creating happy moments. I think you do not have good friends to connect with or a hobby to keep you occupied. Also you should be thinking about an occupation/career which normally scares teenagers. Take a paper and write what made you happy in the past and how would you like to look back at this time, say when you are 25 yrs. Dont worry, its your life and you can make it good.