Why does life’s happiness decline so much after high school?

It doesn't.  It increases exponentially.  Sorry to hear you peaked in high school. Were you one of the kids who won that state-sponsored popularity contest?  Popularity is fleeting.

because of responsibilities , that all no more being protected with your moms and dads , hear comes getting older .

No,.my life just Started after HS. Júnior college, freedom, able to take want classes  I  Wanted, then University, and  work. Money to rent own place, buy cars, motor cycles.

   Freedom

Most of us are much happier after high school. The whole drama, not being able to control hormones, insecurity, fight to establish independence from parents while not really wanting to be independent-  I wouldn't go back to that for any money. in the world! While no one under 30 ever believes this, life gets a lot easier in the 30s. You have a better idea who you are, you should be more established in a career, and you stop worrying about what other people think of you. 

I totally hear you.  You’re looking for a purpose, a community, and a sense of affirmation.  I normally wouldn’t suggest this, but I think you need to find a life coach who specialized in post-adolescents searching for a purpose.

Your social contact has been reduced, we were surrounded with people then.

you're not a kid anymore. you're nearing adulthood and you have to take care of yourself. you have to get a job, to pay for your own place, pay utilities, phone, transportation etc. your life is all about responsibility. fun and happiness will be sporadic, and much appreciated.

I didn't have happiness in HS. I went to prom with my roommates soulmate and I forgot to call back the Spanish girl I liked for the same reason I haven't finished coloring my comic or uploading newer drawings to my site. They did this to a 16 and 17 year old.

I was much happier after high school. 

I don't know I was miserable in high school